Final answer:
In secondary active transport, the simultaneous movement of Na+ and another substance in the same direction is known as symport and cotransport, so the correct answer is D. cotransport AND symport.
Step-by-step explanation:
In secondary active transport, substances are transported across the cell membrane using energy derived from the primary active transport of another substance. When both sodium (Na+) and another substance move in the same direction through a transport protein, this is known as a symport mechanism. An example of this is the Na+/glucose symporter in kidney cells that moves Na+ and glucose into the cell together. Conversely, antiport mechanisms involve the movement of substances in opposite directions, like the sodium-hydrogen ion antiporter that exchanges Na+ for hydrogen ions (H+) across a cell membrane.
Therefore, when another substance moves in the same direction as Na+, the process is correctly referred to as both symport and co-transport. Hence, the correct answer to the question is D. cotransport AND symport.