Hormone responsible for inducing senescence and seed dormancy:
ABA
Cytokinin
Ethylene
Auxin
Senescence is promoted by hormones such as ABA or abscisic acid and ethylene. Bud dormancy refers to the condition when growth, development, and other physical activity are stopped temporarily and growth can be reactivated. This bud dormancy is occurred due to the presence of plant hormone ABA or abscisic acid.
Hence the correct option is a.