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.
