I agree that the correct answer is A.
Given that IAS and aileron deflection are constant it follows that:
The dynamic pressure is constant and therefore roll moment due to aileron deflection is constant.
Given that altitude increased (with IAS constant) it follows that:
TAS increased.
Steady state roll rate is achieved when roll moment is in equilibium with roll damping. And for a given roll rate, roll damping would be less at higher TAS. Therefore roll will go to a higher rate before equilibrium is reached.
As RedBull says, answer C is also tempting because the higher TAS would result in a reduced rate of turn as well. But the question does not specify any angle of bank or even that the aircraft is turning. It could be an aileron roll. So based on the information available in the question there is only one answer that is definitely correct, and that is A.