The answer is D, Why?

The half circle would form a sphere. 

The half square would form a circle if you think about it hard enough think of one of those big cheese circles you see in the dairy section of some supermarkets. 

And the equalaterial triangle would form another cheese wedge but it's heigth would derecrease as going closer and closer to the edge so its heightest point would be the center. 

The triangle as roating on the axis would form a cone