The correct answer is A. ethylene.
Ethylene is a naturally occurring gas that is produced by plants as they ripen. It is also used commercially to artificially ripen green fruit. Ethylene works by stimulating the production of enzymes that break down the cell walls of the fruit, causing it to soften and ripen.
Ethane is a colorless, odorless gas that is lighter than air. It is used as a fuel, a refrigerant, and a solvent. Ethane is not used to artificially ripen fruit.
Carbon dioxide is a colorless, odorless gas that is heavier than air. It is used in fire extinguishers, as a refrigerant, and in the production of soda. Carbon dioxide is not used to artificially ripen fruit.
Acetylene is a colorless, flammable gas that is used in welding and cutting. It is not used to artificially ripen fruit.