The correct option is (a) ABA.
Explanation:
Senescence is an active genetically controlled developmental process in which cellular structure and macromolecules are broken down and translocated away from the senescing organ (typical leaves) to actively growing region that serve as nutrient sinks. Senescence is initiated by environmental cues and is regulated by the hormones, e.g. ABA (Abscisic Acid)
Higher amount of ABA stops protein and RNA synthesis thus accelerating the senescence.
