My aunt Seema was the most generous of all the members of our family. Whenever we went out for dinner,
she always insisted to pay for the meal.

The verb "insist" (when used to express a strong suggestion or demand) is followed by:

“that + subject + base verb”  (subjunctive mood)

OR "insist on + -ing form" (idiomatic prepositional structure of "insist.")

Let us look at each option one by one.

Option (A) that she had paid for the meal :  uses past perfect ("had paid"), which implies the action already happened before some past reference, but that's not what "insisted" conveys. 

“She insisted that she had paid” implies she claimed she had already paid. But the original context is about her insisting on paying, not claiming she did. So, this doesn’t match the intended meaning 

Option (B) that she pay for the meal : follows the correct subjunctive structure: insisted that + [subject] + [base verb]

“She insisted that she pay” = she demanded/suggested that she herself be the one to pay, which is precisely the intended meaning of the sentence. 

Option(C) on and paid for the meal : seems to try to follow the idiomatic form "insist on + -ing", but it’s not correctly constructed.

If it were: “...insisted on paying for the meal,” it would be correct.

But “insisted on and paid” is grammatically broken — two mismatched verbs, no clear object, and wrong structure.

Option(D) that she will be paying for the meal : has a Tense mismatch . The main verb is in the past tense “She always insisted...” ; But “will be paying” is in the future tense, which doesn’t fit with a past-time frame.

Hence, Option B is the correct answer.