A sentence has been given with a blank to be filled with appropriate option. Choose the correct appropriate.

Air Travel is the _________ growing emitter of climate change gases.

fastest

The correct option to fill in the blank is fastest.

The word fastest is the superlative form of the word fast, which means "moving or happening at a very high speed." In the sentence, the speaker is saying that air travel is the fastest growing emitter of climate change gases. This means that air travel is emitting climate change gases at a rate that is faster than any other industry.

The other options are not grammatically correct in the context of the sentence.

• Fasting is the act of abstaining from food for religious or other reasons. It does not make sense to say that air travel is fasting.
• Fastly is an adverb that means "at a fast speed." However, it is not grammatically correct to use the adverb form of the adjective fast in this sentence.
• Fast is the adjective form of the word fast, but it does not make sense to say that air travel is fast. The sentence is already saying that air travel is emitting climate change gases at a fast rate.