Of the following members of the solar system, which is the one that never exhibits phases to us?

Question

A. the moon
B. Venus
C. Mercury
D. Mars

Answer 1

mars

Answer 2

Actually the answer is none of them, even though the answer they are going for is probably Mars.

The Moon shows phases, as anyone with eyes knows. Venus and Mercury also show a complete phase cycle because they are orbiting the sun within Earth's orbit. We therefore see them both when they are on the opposite side of the sun from us (where they appear full) and when they come between us and the sun (where they show crescent phases). Mars, being outside our orbit, never comes between us and the sun, and so therefore can never show a crescent phase to an earthbound observer. However, our orbits are sufficiently close that there are occasions when the dark side of Mars is slightly turned towards us, and so Mars can show a distinct gibbous phase to an observer from Earth. The planets further out are too distant for this ever to occur and so we only ever see them fully illuminated.

So, Mars will be the answer the question is looking for, but that's wrong.