In environmental chemistry, methane is an air pollutant that is significantly produced by which of the following processes?

Introduction / Context:
This question tests your knowledge of environmental chemistry and sources of greenhouse gases, especially methane. Methane is a potent greenhouse gas that contributes significantly to global warming and climate change even though its concentration in the atmosphere is lower than carbon dioxide. Understanding the major natural and human related sources of methane is important in climate science, agriculture, and environmental policy. Examinations often include questions that ask you to identify which industrial or biological processes emit key pollutants, so recognizing that animal digestion is a major methane source is highly relevant.

Given Data / Assumptions:
- The pollutant of interest is methane gas in the atmosphere.
- Several possible processes are listed, including ultraviolet action on nitrogen compounds, fertilizer manufacture, incomplete coal combustion, and digestion of food by animals.
- You are expected to identify the dominant methane producing process among the options given.
- The focus is on typical textbook level sources, not rare or highly specialized reactions.

Concept / Approach:
The core concept is that methane is produced by anaerobic decomposition of organic matter, especially in the digestive systems of ruminant animals such as cattle, sheep, and goats. Microorganisms in the stomachs of these animals break down plant material and generate methane as a by product, which then escapes through belching and other processes. While methane can also originate from wetlands, landfills, and some industrial activities, the most prominent and widely taught source in basic environmental science is enteric fermentation in animals. Therefore, you must connect the idea of animal digestion with methane generation and distinguish it from processes that mainly produce other pollutants such as nitrogen oxides or carbon monoxide.

Step-by-Step Solution:
Step 1: Recall that methane, CH4, is a colorless, odorless gas that acts as a strong greenhouse gas in the atmosphere. Step 2: Remember that one of the largest natural and agricultural sources of methane is microbial activity in the digestive systems of ruminant animals, a process called enteric fermentation. Step 3: Examine each option and match it with its typical emissions. Ultraviolet action on nitrogenous compounds is more closely associated with photochemical smog and nitrogen oxides. Step 4: Manufacturing ammoniacal fertilizers and burning coal in insufficient air are processes more commonly associated with emissions of oxides of nitrogen, carbon monoxide, and other gases, not primarily methane. Step 5: Conclude that digestion of food by animals, especially ruminant livestock, is the main methane producing process in the given list, so that option is correct.

Verification / Alternative check:
To verify this reasoning, you can refer to climate change reports or educational summaries that list the main anthropogenic sources of methane. These sources consistently emphasize agriculture and livestock, particularly ruminants, as major contributors to methane emissions. Wetlands and rice paddies are also listed, but those are not among the options here. Industrial processes like fertilizer manufacturing may involve ammonia and nitrogen compounds, but they are not highlighted as dominant methane emitters. Therefore, cross checking with reliable environmental data supports the conclusion that animal digestion is the correct answer.

Why Other Options Are Wrong:
The action of ultraviolet light on nitrogenous compounds leads mainly to the formation of nitrogen oxides and contributes to photochemical smog, not primarily methane. The manufacture of ammoniacal fertilizers deals with ammonia and related nitrogen species rather than methane emission as a central issue. Burning coal in insufficient air generates carbon monoxide and soot particles but not large amounts of methane. While small amounts of methane may occur in some combustion processes, the textbook emphasis is on the greenhouse gas release from enteric fermentation in animals. Hence, these options do not represent the main methane source and are considered incorrect.

Common Pitfalls:
Students often assume that any industrial process, such as fertilizer production or coal burning, must be the correct answer because these are obviously polluting activities. This leads them to overlook biological sources like animal digestion, which also play a huge role in greenhouse gas emissions. Another common mistake is not distinguishing between different pollutants and simply guessing based on which option sounds most technical. To avoid such errors, it is important to connect each process with its primary pollutants and to remember that agriculture and livestock contribute significantly to methane in the atmosphere.

Final Answer:
Methane as an air pollutant is significantly produced by digestion of food by animals.