Name the plant hormones responsible for the following:
$(a)$ Elongation of cells
$(b)$ Growth of stem
$(c)$ Promotion of cell division
$(d)$ Falling of senescent leaves.

(N/A) Auxin is responsible for cell elongation.
$(b)$ Gibberellin is responsible for the growth of the stem.
$(c)$ Cytokinin is responsible for the promotion of cell division.
$(d)$ Abscisic acid is responsible for the falling of senescent leaves.