Choose the grammatically correct sentence:

The correct answer is Option (1) → The blue bicycle is ours, but the helmet hanging on it is hers.

Possessive pronouns have no apostrophe:

“Ours” and “hers” are possessive pronouns. They mean belonging to us and belonging to her. Possessive pronouns never take an apostrophe. Both are written correctly here, and the sentence has no other error.

Why the Other Options are Incorrect?

Option 2: Ravi and myself will set up the chairs for the parents’ meeting. — 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: Ravi and I will set up the chairs for the parents’ meeting.

Option 3: One must keep your promises to friends. — Incorrect. The sentence begins with “One”, so the later pronoun must also be a form of “one”. It cannot switch to “your”.
Corrected: One must keep one’s promises to friends.

Option 4: Mr Bose is the neighbour whom fixed our broken gate last week. — Incorrect. The given sentence says “the neighbour whom fixed”. This is wrong: the neighbour did the fixing, so the subject form “who” is needed. “Whom” is the object form.
Corrected: Mr Bose is the neighbour who fixed our broken gate last week.