I think this the example where we need to Understand that GMAT accepts a little ambiguity in the pronoun reference.
As in option
C) Using a metal detector, hobbyists can locate old coins and other valuables even though
they are buried in the sand and dirt.----
Here they cannot logically refer to the hobbyists is understood but here as well "they" can refer to only old coins (plural) or only valuables ( plural) or both .
D) Buried in the sand and dirt, old coins and other valuables can be located by hobbyists
using a metal detector.
---here the modifier "Buried in the..blabla " refers to only old coins but the intended meaning wants "both old coins and other valuables" to be buried

so best option between C and D would be C
Right Experts??