Not being an aerodynamicist I'd have gone for (D). I remember exactly this exercise in the Hawk when an instructor tried to convince me that the highest rate of roll occurred at altitude, when it clearly didn't. On landing I found the reference in the manual which said the optimum rate of roll was at sea level. I always assumed he had written the lesson down incorrectly on his knee pad, but who knows.
The clue might be in the 'constant IAS' phrase, you get better roll rates at high IAS at sea level but you can't make those high IAS's at height because of Mach effects. I will wait for an aerodynamics expert...