Read the passage about a climate agreement carefully and choose the verb that correctly completes the line whose aggregate greenhouse gas (GHG) emissions ______ at least 55 per cent of global emissions.

Introduction / Context:
This question is the final part of the cloze passage on the climate agreement. The sentence describes countries whose aggregate greenhouse gas emissions make up a certain share of total global emissions. The verb in the blank must express the idea of forming or making up at least 55 per cent of the world's emissions. This type of wording appears frequently in environmental reports and economic analyses, so recognising the correct verb is very useful for comprehension as well as exam success.

Given Data / Assumptions:

• Sentence fragment: whose aggregate greenhouse gas (GHG) emissions ______ at least 55 per cent of global emissions.
• Options: established, constitute, enact, complement.
• The subject is aggregate GHG emissions of the ratifying countries.
• The blank needs a present tense verb that links emissions to a percentage of global emissions.

Concept / Approach:
The verb constitute means to be or to form, especially when talking about parts making up a whole. When we say emissions constitute 55 per cent of global emissions, we mean they make up that portion of the total. Established usually refers to setting up institutions or proving facts. Enact is used for passing laws or legislation. Complement means to complete or go well with something, not to form a percentage of a total. Therefore, constitute is the only verb that correctly and precisely expresses the intended meaning in this scientific and legal context.

Step-by-Step Solution:
Step 1: Understand that the sentence describes the share of global emissions represented by certain countries. Step 2: Recognise that the verb must mean make up or account for a percentage. Step 3: Evaluate established. It usually means set up or proved, which does not fit this structure. Step 4: Evaluate constitute. It directly means form or make up and is widely used in such statistical expressions. Step 5: Evaluate enact and complement, both of which relate to laws or supportive relationships, not numerical shares.

Verification / Alternative check:
Read the sentence with each option. Emissions established at least 55 per cent of global emissions is ungrammatical. Emissions enact at least 55 per cent of global emissions does not make sense. Emissions complement at least 55 per cent of global emissions also fails to convey a logical idea. In contrast, emissions constitute at least 55 per cent of global emissions is grammatically correct and matches standard usage in climate and economic reports. This confirms that constitute is the correct choice.

Why Other Options Are Wrong:

• established: Refers to setting up, founding or proving, and does not link a subject to a share of a total in this way.
• enact: Used for making laws, passing acts or regulations; it does not describe numerical composition.
• complement: Means to complete or enhance something else, often used with colours or skills, not with percentages of emissions.

Common Pitfalls:
Students may mistake established for a general word indicating importance or significance and surprisingly choose it, but they overlook the exact grammatical requirement. Another pitfall is to choose a sophisticated sounding verb like complement without checking its core meaning. To avoid such errors, always match the verb with the type of object and structure in the sentence. Whenever you see a phrase with a percentage of a total, verbs like constitute, account for or represent are far more likely to appear than the other alternatives.

Final Answer:
The correct verb is constitute, giving the line whose aggregate greenhouse gas (GHG) emissions constitute at least 55 per cent of global emissions.