Complete the sentence. If he ______ more careful he would not have made that mistake.

had been

will be

was

If + past perfect (had + past participle) → would have + past participle

"If he had been more careful, he would not have made that mistake." (Correct)
(B) will be (Future tense does not fit a past hypothetical situation)
(C) is (Present tense does not fit with "would not have made")
(D) was (Incorrect tense; should be "had been" for past unreal situations)