-  a will be used before singer.

The given sentence ‘On my request Jatin introduced me to his friend who is singer and scientist’ is grammatically incorrect. The error lies in the fourth part of the sentence ‘who is singer and scientist’.

The correct usage of a noun phrase should be ‘who is a singer and a scientist’. The correct usage of the noun phrase should be ‘a singer’ and ‘a scientist’ as both of them are singular nouns. The article ‘a’ should be used before the noun and an ‘and’ should be placed before the conjunction.

Hence, option D - 4 is the correct answer.

Relevant Definitions and Formulas:
Noun Phrase: A noun phrase is a phrase that has a noun as its head or the main word along with its modifiers.

Singular Noun: A singular noun is a word that names only one person, place, thing, or idea.

Conjunction: A conjunction is a word that joins two or more words, phrases, or clauses together.

Article: An article is a word that is used before a noun to indicate the type of reference being made by the noun.