Well, the diameter of a circle is simply a length, so your measurement will have units of length. We just have to find an answer that has only units of length.
A). gram, second . . . mass and time. That can't be it.
B). kilogram, ampere . . . mass and current. That can't be it.
C). centimeter, meter . . . both lengths. This one is looking good.
D). candela, mole . . . light intensity and some chemical thing. That can't be it.
So it can't be anything else on this list but C .
