Explanation:

This needs to be attempted carefully. There is 'a' before the blank and plural 'tests' after the blank so the word in the blank should express a collective to make the sentence grammatically correct.

Among the given options, only series fits in grammatically as well as contextually. 'A large of tests' doesn't make any sense. One doesn't undergo a design of tests or a booklet of tests or a delegation of tests.

Hence, these are incorrect as well.