When we see saturn going through a period of apparent retrograde motion, it means (a) saturn is
temporarily moving backward in its orbit of the sun. (b) earth is passing saturn in its orbit, with both
planets on the same side of the sun. (c) saturn and earth must be on opposite sides of the sun.

b) Earth is passing Saturn in its orbit, with both planets on the same side of the Sun.

Explanation:

Retrograde motion is an optical effect due to the fact that Earth rotates more quickly than the planet that apparently has a retrograde motion in the sky.

For this particular case, Saturn has a slower speed in its orbit around the sun. That means that the Earth will pass it, and that will give the effect that the planet is moving backward. That same scenario can be seen between two cars on a highway, the faster car will see the slower car when it passes going in backward for a short fragment of time.

Remember that all the object in the night sky moves from east to west (since the Earth rotates counterclockwise in its daily movement), in the case of a planet with retrograde motion, it will move from west to east for a couple of months.        

your answer is c! hope this : )