Which one of the following is not a second messenger in hormone action?
a)	GMP	b)	Calcium	c)	Sodium	d)	AMP

(c)

Second messengers are the organic molecules and sometimes the metal ions, acting as intracellular signals,  whose production or release usually amplifies a signal such as a hormone, received  at the cell surface.

Sodium (Na) is not a second messenger in hormone action.