The smallest unit of element is

Atoms

An atom is the smallest unit of a pure substance or element that can exist and still retain the properties of the original substance or element. Key concepts: common substances, such as air, water, wood and cloth are mixtures of materials and can be chemically or physically broken down into simpler substances.

endo:

regulates body hormones, aldosterone, vasopressin

neural:

signal, regulates body using electric signals, reflex

endo has to do with hormones; aldosterone and vasopressin are both hormones. i also had this question and got it right : d