Question

Transitions between the juvenile, reproductive and senescent phases in both plants and animals is maintained by

Answer 1

Enzymes
Enzymes are proteins which catalyze biochemical reactions occurring inside the body.

Hormones
Hormones are intercellular messengers produced by endocrine glands that play a role in regulating the physiological processes of our body such as reproduction, metabolism, sleep, etc.
The transition between the juvenile, reproductive and senescent phases in the life of a sexually reproducing organism are largely influenced by hormones. These chemicals influence the external and internal changes that the body undergoes while attaining sexual maturity, the reproductive cycles, metabolism, etc.

Vitamins
Vitamins are organic substances needed in trace amounts for normal growth and immunity.

Final answer
(b) Hormones