Reasonably basic question. Answer should be D.
IAS is a measure of the amount of air molecules flowing past you. Because of decreased air density (read air molecules), to maintain the same IAS at altitude you'll need to fly faster. Along with this, increased thrust is also required at altitude because the engine is less efficient (less power), because you need to travel faster to create the same amount of lift, and because the prop is creating less thrust too.
Answer is D