imo:D

We are in need of Present perfect tense...

so we use Has+past participle..

As the non-underlined sentence has a verb in present tense, we will keep the whole sentence in the present tense..

A.she tells him to stop calling, but he acted as if he had not understood her.

past and past perfect tense used

B. she told him to stop calling, but he acted as if he had not understood her.

past and past perfect used

C. she tells him to stop calling, but he acts as if he did not understood her.

past tense used

D. she tells him to stop calling, but he acts as if he has not understood her.

E. she tells him to stop calling, but he acted as if he does not understand her.

use of two different tenses in one sentence.

thanks,
i hope this helps.

got it thanks!