When does the cell have to use active transport?
a. when the cell is low on energy
b. when the cell is in a highly concentrated solution
c. to move molecules across the membrane quickly
d. to move molecules across the membrane against a gradient

The best answer is D.

In active transport, molecules move against a concentration gradient from areas of lower concentration to areas of higher concentration. This happens a lot in neurons. The membrane proteins are constantly pumping ions in and out  to get the membrane of the neuron ready to transmit electrical impulses.

In active transport , energy is required to move molecules across the cell membrane. Carrier proteins are needed for this e.g. proteins of the GLUT family which transport glucose molecules across the cell membrane. Carrier proteins are very specific. GLUT proteins will only move glucose molecules and not sodium or calcium. There are hundreds of types of carrier proteins.

dude i stopped reading after the first 7 words but hey you seem real smart thats awesome have a great day