Final answer:
When the number of rubber bands is doubled from two to four and all other conditions remain constant, the acceleration of the object would double as well, making the correct answer c) 2a.
Step-by-step explanation:
If two rubber bands cause an object to accelerate with acceleration a, using four rubber bands of the same type would presumptively double the net force on the object, because each pair of additional rubber bands should contribute a similar amount of force as the first pair, assuming they're of the same type and applied in a way that the forces are additive and no other factors (like friction or air resistance) have changed.
Therefore, if the mass of the object remains constant, the acceleration of the object would also double, according to Newton's second law (F=ma), because the net force is doubled while the mass remains the same.
Thus, the correct answer would be c) 2a.