Read the given sentences carefully and choose the grammatically incor…

The correct answer is Option (3) → C and D only

Note: The given answer is as per the official NTA Answer Key. However, there appears to be a discrepancy in the key: the correct answer should be Option (1) → A, B and D only, as explained below.

A. After Sir Davos Seaworth was punished for smuggling, he had three less fingernails to clean. — Incorrect. Fewer is used with things that can be counted (fingernails, people, books); less is used with amounts that cannot be counted (water, time, money).
Correct form: After Sir Davos Seaworth was punished for smuggling, he had three fewer fingernails to clean.
Example: This year there were fewer accidents on the highway.

B. I am irked every time my cousin introduces me as her older sister; I am only six months older than her! — Incorrect. In formal grammar, "than" here is followed by a clause that is understood ("than she is"), so the subject pronoun is needed; "than her" is common in speech but is not the correct written form.
Correct form: I am irked every time my cousin introduces me as her older sister; I am only six months older than she is!
Example: My brother is taller than I am.

C. My brother is an introvert and struggles with social interactions, so he has a few friends. — Correct. The sentence is grammatically well formed. A few means "some", while few means "hardly any"; "few friends" would suit the sense of "struggles" better, but the choice is one of meaning, not a grammatical error.
Example: We have a few minutes left, so let us revise once more.

D. The team decided to stop for a meal while they awaited farther instructions from the head office. — Incorrect. Farther is used only for physical distance; further also means "more" or "additional".
Correct form: The team decided to stop for a meal while they awaited further instructions from the head office.
Example: Please wait for further orders.