We can answer this question by looking at Newton's second law:

which can be rewritten as

where F is the force experienced by the rocket, m is its mass, a is its acceleration. In the rocket's case, the mass of the rocket decreases (because the fuel in the tank decreases), while the force remains constant, so the ratio F/m increases, and therefore the acceleration of the rocket increases.
Therefore, the correct answer is
D - The acceleration increases because the mass decreases.