Denying that one of
its many irregularities had been the long-term mismanaging of company funds, the AD & M company produced clear evidence to back its claim.
The sentence is speaking about AD&M Company, which is singular. So we can immediately chop out option D and E because of these options using plural pronoun 'their'. Tricky option A and B. But if you understand the meaning of the sentence, answering it would be a cake walk
“
Had been” means something began in the past, lasted for some time, then ended. This is entirely in the past. This verb tense is known as past perfect. Example: He had been in prison from 2000 to 2005.
Both “
has been” AND “
have been” mean something began in the past and has lasted into the present time. Example: He has been in prison since 2010.
A.
its many irregularities
had been the long-term -> 'had been' usage is correct her.
B. its many irregularities
has been the long-term -> 'has been' usage is wrong because the irregularities.....is not continuing till date.
C. its many irregularities
is the long-term -> they are talking about denying something that happen in past. Tense error.
D.
their many irregularities is the long-termE.
their many irregularities had been the long-term