A volatile substance released by ripened oranges that hastened the ripening of stored bananas was later identified as:

The correct option is Ethylene.

Explanation

The question refers to the historical identification of the specific plant growth regulator (PGR) responsible for inducing fruit ripening.

Detailed Analysis:

• Historical Context: In 1910, H.H. Cousins observed that a volatile substance released from ripened oranges hastened the ripening of stored unripened bananas.
• Identification: This volatile substance was later chemically identified as Ethylene ($C_2H_4$).
• Function: Ethylene is a simple gaseous plant hormone. It is synthesized in large amounts by tissues undergoing senescence and by ripening fruits. It accelerates the ripening process by enhancing the rate of respiration, a phenomenon termed the respiratory climacteric.
• Other Options: Carbon dioxide is often used to retard ripening in storage, while Methane and Nitric oxide are not the primary hormones responsible for fruit ripening.

Key Takeaway: Ethylene is the only gaseous plant hormone and is the primary agent responsible for fruit ripening and the regulation of senescence in plants.