Answer
c.
Work Step by Step
Because the sodium concentration in the cell is much less than the surrounding fluid, in order to force even more sodium into the surrounding fluid, you would need to pump it out of the cell. If sodium were to follow its concentration gradient, then sodium would enter the cell instead of exit it. To force sodium out of the cell, the sodium pump would need energy to fuel it. This is considered active transport because the pump is moving a solute (sodium) against its concentration gradient.
