In the mechanism of action of a protein hormone, one of the second messengers is

1. Cyclic AMP

2. Insulin

3. T3

4. Gastrin

(a) Cyclic AMP is one of the second messengers involved in the action of a protein hormone. The other three options are incorrect as
Insulin regulates glucose homeostasis.
T3 regulates the metabolic rate of the body and thus, helps in maintaining Basal Metabolic Rate (BMR).
Gastrin is a peptide hormone, that stimulates the secretion of hydrochloric acid and pepsinogen.