Biology

The correct answer is C.an expenditure of energy.

Explanation:

Active transport is a process that moves molecules against their concentration gradient (from low concentration to high concentration), and this process requires energy, usually in the form of ATP.

Why the other options are incorrect:

A.The movement of molecules from high concentration to low concentration: This describes passive transport, not active transport. Passive transport moves molecules down their concentration gradient (from high to low concentration) without the need for energy.

B.A process which occurs only in the cells ...