What is the source of this question?
The intended meaning of the sentence seems to be that, "even though" coins and valuables are buried, they can be located. A contrast is implied.
C maintains that contrast, and uses the active voice (preferred). "They" does seem to be a slightly ambiguous pronoun reference. In such cases, the pronoun will refer to the nearest noun (unless that noun is used to qualify some other word).
D removes that contrast. D is essentially saying:
Old coins and other valuables are buried in the sand.
Old coins and other valuables can be detected by hobbyists using a metal detector.
Some phrase suggesting contrast should have been part of the opening modifier.
There is another, more subtle, error in D. In "hobbyists using a metal detector", there is no comma after "hobbyists". The construction suggests that "only" hobbyists "who use" a metal detector can detect the coins, and not the intended "ALL hobbyists can detect the coins BY USING a metal detector".
--Prasad
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