D as well...
a. Last August, while cleaning the field, a mine exploded.
who was cleaning the field?
b. While clearing the field, a mine exploded last August.
same problem as in A...subject is missing.
c. Last August, a mine exploded while clearing the field.
subject is missing again.
d. A mine exploded last August while the field was being cleared.
subject here is field
and we have the verb too was being cleared
Futher this option has the proper tense: was being
- past continious. There is also an action going on at the same time: A mine exploded. Last August qualifies past tense.
e. A mine was exploded by clearing the field last August
changes the meaning of the sentence.
Wish you all good luck