The correct answer is (C) who.
The sentence is about students who scored more than 90% marks in class XII examination of CBSE. The students are the subjects of the sentence, and the President of India is the object. The verb “invited” requires a subject, so the answer must be a pronoun that refers to the students. Of the four options, only “who” is a subject pronoun.
“Whom” is an object pronoun, so it cannot be used as the subject of a sentence. “That” and “which” are relative pronouns, and they can be used to refer to both subjects and objects. However, in this case, they would be ambiguous, because they could refer to either the students or the President of India. “Who” is the only option that clearly refers to the students.
Here is a diagram that shows the grammatical structure of the sentence:
[All the students] [who scored more than 90% marks in class XII examination of CBSE] [were invited to a tea party by the President of India].
The subject of the sentence is “all the students.” The verb is “were invited.” The object of the verb is “to a tea party by the President of India.” The relative clause “who scored more than 90% marks in class XII examination of CBSE” modifies the subject “all the students.”
I hope this explanation is helpful!