Which statement would be true of a membrane-bound protein that works in conjunction with a sodium–potassium pump during secondary active transport?
a. it will be able to transport an ion in both directions across the cell membrane and a small molecule in one direction.
b. it will block a small molecule from moving across the cell membrane in response to an increase in concentration of an ion in the external environment.
c. it will move a small molecule across the cell membrane in response to a build-up in concentration of the molecule in the external environment.
d. its conformation will change when both an ion and a small molecule bind to it.

The correct answer is option d. "Its conformation will change when both an ion and a small molecule bind to it".

Explanation:

The secondary active transport is a form of active transport at which a ion (typically Na+ or H+)  is transferred down its electrochemical gradient to the uphill movement of another molecule. Secondary active transport takes place by the conjunctive work of membrane-bound proteins and sodium–potassium pumps. During secondary active transport, both of these proteins change its conformation when an ion and a small molecule bind to it. This change in conformation makes possible that the ions and the small molecules pass through the membrane.