Final answer:
All forms of active transport, along with diffusion, facilitated diffusion, osmosis, and isotonic, rely on concentration gradients for their occurrence.
Step-by-step explanation:
All forms of active transport, along with diffusion, facilitated diffusion, osmosis, and isotonic, have a common characteristic, which is that they rely on concentration gradients for their occurrence. These processes involve the movement of substances from an area of higher concentration to an area of lower concentration. Unlike passive processes like diffusion and facilitated diffusion, active transport requires energy input in the form of ATP to move substances against their concentration gradient. Thus, the correct answer is d. They rely on concentration gradients.