In urban environmental studies, which one of the following gases is released in the largest quantity from landfills in towns and cities?

Introduction / Context:
Landfills are sites where municipal solid waste is dumped and compacted. Much of this waste is organic, including food scraps, paper, and garden waste. When such organic matter decomposes under conditions that are often low in oxygen, gases are produced. This question tests your knowledge of which gas is predominantly released from landfills and why it is important for both environmental pollution and climate change.

Given Data / Assumptions:

• The context is landfills in urban areas, where mixed municipal waste is stored.
• The options include nitrogen, hydrogen, methane, and oxygen as possible major gases released.
• It is assumed that decomposition processes occur largely under anaerobic or low oxygen conditions inside the landfill body.

Concept / Approach:
Landfills containing organic waste develop zones where oxygen is scarce because microbial respiration and the covering of waste layers quickly consume available oxygen. In these anaerobic conditions, microorganisms break down organic matter and produce landfill gas. This gas typically consists of roughly 50 to 60 percent methane (CH4) and 40 to 50 percent carbon dioxide (CO2), plus trace amounts of other gases. Methane is a flammable gas and a potent greenhouse gas with a global warming potential much higher than carbon dioxide over shorter time scales. Nitrogen and oxygen are abundant in the atmosphere but are not the main products of anaerobic decomposition in landfills. Hydrogen may be produced in small quantities but is not the dominant gas, and oxygen is actually consumed rather than released inside landfill layers. Therefore, methane is the gas released mostly from landfills in urban areas.

Step-by-Step Solution:

Verification / Alternative check:
Reports on landfill management and environmental engineering textbooks describe landfill gas composition as being dominated by methane and carbon dioxide, typically around 50 to 60 percent methane. Landfill gas collection systems are often installed to capture methane for flaring or energy production, precisely because methane emissions from landfills contribute significantly to greenhouse gas inventories. This practical emphasis on methane in landfill management confirms that methane is the main gas of concern and the correct answer in this question.

Why Other Options Are Wrong:

Nitrogen, the main component of the atmosphere, is wrong because nitrogen is not produced in large quantities by the decomposition of waste; it is mostly just present as part of surrounding air.

Hydrogen, a light combustible gas, is incorrect because although small amounts may form during certain decomposition processes, hydrogen is not the dominant component of landfill gas.

Oxygen, released continuously from decomposing garbage, is wrong because oxygen is actually consumed inside landfills by aerobic microbes and is quickly depleted; it is not a principal gas emitted.

Common Pitfalls:
Students sometimes choose nitrogen or oxygen because they associate these gases with air without considering the specific chemical processes occurring inside a landfill. Another common mistake is not remembering that anaerobic decomposition of organic matter generates methane. To avoid such errors, link this topic with anaerobic digestion used in biogas plants: both biogas and landfill gas are rich in methane produced during the breakdown of organic material in low oxygen conditions. Thus, methane is the correct choice here.

Final Answer:
Hence, the gas released mostly from landfills in urban areas is methane, a flammable greenhouse gas generated during decomposition of organic waste.