<u><em>C: </em></u><em>The D Chord uses only four strings. </em>
Explanation:
It can't be A because the passage says you may rest your thumb on the 5th string. B is a possible answer but it gives an implicit choice, not explicit. D says the D Chord uses all six strings when the passage states to avoid playing the fifth and sixth strings.
Therefore, this evidence supports our answer that <u><em>C</em></u> is correct.