Step 1: Understanding the Concept:
Plant growth regulators (PGRs) are organic compounds that modify physiological processes in plants.
They can be categorized into plant growth promoters and plant growth inhibitors/regulators.
Step 2: Detailed Explanation:
Ethylene is a natural, gaseous plant hormone synthesized in ripening fruits, senescing flowers, and vegetative tissues.
Its primary agricultural application is promoting rapid, uniform ripening of climacteric fruits (such as bananas, mangoes, and tomatoes).
It also accelerates senescence and abscission of leaves and fruits.
Step 3: Final Answer:
The primary use of ethylene gas is the ripening of fruits, which is option (C).