Choose the grammatically correct sentence:

The correct answer is Option (3) → The rooms of the new hostel are bigger than those of the old one.

“Those of” for a plural noun in a comparison:

The sentence compares the rooms of two hostels. To avoid saying “the rooms” twice, it uses “those of”. “Those” stands for a plural noun, and “rooms” is plural. So “those of” is correct.

Why the Other Options are Incorrect?

Option 1: My cousin and myself will decorate the stage for the annual day. — Incorrect. The given sentence uses “myself” as part of the subject. “Myself” cannot be the subject of a verb. The subject form “I” is needed.
Corrected: My cousin and I will decorate the stage for the annual day.

Option 2: Us students were asked to wait outside the exam hall. — Incorrect. The given sentence uses “us” as the subject. “Us” is the object form. The subject needs the subject form, “we”.
Corrected: We students were asked to wait outside the exam hall.

Option 4: The scooter which my uncle sold it to us last week still runs well. — Incorrect. The given sentence has an extra “it”. “Which” already stands for the scooter, so “it” must go.
Corrected: The scooter which my uncle sold to us last week still runs well.