Once the cache of arms was found stashed behind the house, the police inspector
did not doubt whether the man they arrested was the mastermind behind the serial bombing.
A.
did not doubt whether the man they
(Past Participle missing) arrested (did used for work done, while the verb is been used later had is better choice here because it shows what they already done, since the passage talks about the work being done so Had is a better word choice)
B. had no doubt that the man they
had arrested (correcting all mistake)
C. had not doubted that the man they
(Past Participle missing) arrested
D. has not got any doubt if they man they
(Past Participle missing) arrested (too wordy)
E. had no doubt whatsoever about the man they
(Past Participle missing) arrested
Since the whole statement describe a past event, it has to be formulated in past tense only