Final answer:
Simple diffusion is the transport mechanism that occurs without coupling the movement of the solute to a second solute, as it relies solely on the concentration gradient and does not require energy or assistance from other molecules.
Step-by-step explanation:
The mechanism that occurs without coupling transport of the solute to the movement of a second solute is D) Simple diffusion. Simple diffusion is a process whereby small molecules or ions move across a cell membrane without the input of energy by the cell, and without the assistance of any other solute. It is driven solely by the concentration gradient, with particles moving from an area of higher concentration to one of lower concentration. In contrast, facilitated diffusion, although also a passive transport process, involves the use of transmembrane proteins to help transport solutes across the plasma membrane.
Primary active transport and secondary active transport both involve the active movement of solutes against their concentration gradient. Primary active transport is directly powered by ATP, while secondary active transport relies on the electrochemical gradient created by primary active transport.