They first sun-dried the garbage for one to three days to bring down the moisture level.

The subject and the object should be interchanged. Also, the doer of the action can be omitted in passive voice. 
'The garbage' is the object in the given sentence and hence, the 'They first sun-dried the garbage' will be changed to 'The garbage was first sun-dried'. The other 2 parts of the sentence 'for one to three days' and 'to bring down the moisture level' do not require any change. Therefore, the passive voice of the given sentence is 'The garbage was first sun-dried for one to three days to bring down the moisture level' and hence, option D is the right answer.