In the following sentence supply a verb in agreement with its subject:
Ninety rupees {Fill_Regular} too much for this bag.

Knowing the subject-verb agreement helps us understand the fundamental structuring of a sentence. This agreement or concord registers how a word changes its singular-plural form betting on the opposite words or tense associated with it.
According to the agreement, the verb following a singular noun should be plural which following a plural noun should be singular. the identical is applicable vice-versa too.
That being the case, we understand that the topic or noun within the given sentence is ‘ninety rupees’ which is plural within the count. Therefore, we identify that the verb following this may be ‘is’ and not ‘are’.
Option (a) has ‘is’ in it and hence, option a is that the right answer.
Option (b) is wrong as ‘are’ is that the descriptor of ‘is.’ This cannot accompany the plural subject so it's incorrect here.
Similarly, ‘were’ in option (c) is that the tense of ‘are’ and clearly cannot suit the given sentence.
Option (d) is additionally wrong. Though ‘be’ may be a style of is, are, and were; it can not be the proper choice here.
Hence option (a) is correct.