Retrograde motion is observed because planets

Question

A orbit at different speeds.

B orbit in two directions.

C orbit backwards.

D rotate as they orbit

Answer 1

A - and only planets farther from the sun than earth exhibit this phenomenon.

Answer 2

All planets orbit the sun in the same direction.

So B and C are ruled out.

The rotation of a planet as it orbits has nothing to do with how we see the planet from Earth, so that rules out D.

The farther from the sun, the slower the planet moves in its orbit.  So that means the answer must be A.