The correct answer is (c). Ethylene is a plant hormone that is mainly responsible for fruit ripening. It is produced by the fruit itself as it matures, and it triggers a number of changes that lead to the softening, color change, and flavor development of the fruit. Ethylene can also be artificially applied to fruits to speed up the ripening process.
Cytokinins are plant hormones that promote cell division and growth. They are not directly involved in fruit ripening.
Abscisic acid (ABA) is a plant hormone that is involved in a number of processes, including seed dormancy, leaf abscission, and stress responses. It is not directly involved in fruit ripening.
(d) None of the above is also not the correct answer.