The correct answers are:
A) the number of different colors on the page; and
D) half the speed of the car.
Explanation:
A variable represents an unknown amount.
For A, we do not know how many colors are on the page; this means we would use a variable to represent it.
For D, while we know we want half of the speed of the car, we do not know what the speed of the car actually is; therefore we would use a variable to represent it.