Interaction between ...... and certain environmental factors regulate the reproductive processes and the associated behavioural expression of organisms.

The correct option is A Hormones

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 hormones along with the environmental factors influence the morphological, behavioural and physiological changes that the sexually reproducing organisms experience during different phases of their lives. All events involved in reproduction, starting from the developmental changes for sexual maturity, behavioural changes during puberty or during the breeding seasons, changes occuring in the reproductive organs during the reproductive cycles, mating, development of embryo, its birth, etc are influenced by hormones.

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

Final answer
(b) Hormones