A passage with three blanks is given below, followed by options containing a single word. Choose the one word that can correctly fill all the three blanks. Emission ______ are the legal requirements governing air pollutants released into the atmosphere. Emission ______ set quantitative limits on the permissible amount of specific air pollutants that may be released from specific sources over specific timeframes. Many emissions ______ focus on regulating pollutants released by automobiles and other powered vehicles, as well as from industries and power plants.

Introduction / Context:
This question is a vocabulary and reading comprehension item based on a short passage about environmental regulation. The passage contains three blanks, and you must choose a single word that fits naturally in all three positions. The text discusses legal requirements related to air pollution, so you need to focus on collocations (usual word combinations) used in environmental law and policy, rather than only on isolated dictionary meanings.

Given Data / Assumptions:

• The passage talks about legal requirements governing air pollutants.
• The first blank refers to a set of legal requirements that control emissions.
• The second blank refers to something that sets quantitative limits on emissions.
• The third blank refers to many such sets that focus on different pollution sources, such as automobiles and industry.
• The options given are: Norms, Pollutants, Factors, Standards.

Concept / Approach:
In environmental law and policy, the most common phrase is "emission standards". These standards are specific numeric limits set by authorities to control how much of a certain pollutant can be released from a given source. The word "standards" can logically represent a set of quantitative limits and is widely used in official documents. "Norms" is also sometimes used informally, but "emission norms" does not fit as smoothly in all three places and is less universal. "Pollutants" and "factors" do not describe legal requirements; instead, they describe what is being controlled (pollutants) or influences on a process (factors).

Step-by-Step Solution:
Step 1: Insert "standards" in the first blank: "Emission standards are the legal requirements governing air pollutants released into the atmosphere." This is a known formal phrase and reads correctly.Step 2: Insert "standards" in the second blank: "Emission standards set quantitative limits on the permissible amount of specific air pollutants." This matches the idea that a standard gives a numeric limit.Step 3: Insert "standards" in the third blank: "Many emissions standards focus on regulating pollutants released by automobiles and other powered vehicles." Again, this is natural and reflects real regulatory practice.Step 4: Test the other options in each blank. "Emission norms" is somewhat acceptable in the first use, but "Emission norms set quantitative limits" is less typical than "standards", and "Many emissions norms focus" sounds awkward and less standardised in formal English.Step 5: Confirm that "pollutants" and "factors" cannot describe legal requirements, so they fail in the first blank itself.

Verification / Alternative check:
Think about real world phrases you see in news articles and government documents: "Euro emission standards", "Bharat Stage emission standards", "vehicle emission standards". These are all standard usage. It is very rare to see "emission factors" used in this context (that phrase refers to coefficients used in calculations, not legal rules), and "emission pollutants" is grammatically wrong. Thus, "standards" is the only option that preserves both grammar and common usage in all three positions.

Why Other Options Are Wrong:
"Norms" can sometimes mean accepted rules, but "emission norms are the legal requirements" is less precise and less formal than "standards", and the phrase "emissions norms focus on regulating pollutants" is not the standard collocation in technical English. "Pollutants" names the harmful substances themselves, which clearly cannot be "legal requirements". "Factors" refers to influences or elements in a calculation, not to official limits imposed by law, so it does not fit any position.

Common Pitfalls:
A common mistake is to choose "norms" because it is often used in everyday speech about rules. However, exam questions like this usually prefer the technically precise term that matches real policy language, which is "standards". Another mistake is to evaluate only the first blank and ignore the requirement that the same word must fit all three places. Always test the chosen word in every blank before deciding.

Final Answer:
The word that correctly completes all three blanks is Standards, so option D is correct.