Choose the grammatically correct sentence:

The correct answer is Option (4) → The cricket coach whom every boy in our colony respects is retiring next month.

“Whom” as the object in a relative clause:

“Whom” is the object form. It is used when the person receives the action in the clause. Here every boy respects the coach, so the coach is the object of “respects”. “Whom” is right, and the sentence has no other error.

Why the Other Options are Incorrect?

Option 1: This tiffin box is your’s, and the steel one is mine. — Incorrect. The given sentence writes “your’s”. Possessive pronouns never take an apostrophe, so the form is “yours”.
Corrected: This tiffin box is yours, and the steel one is mine.

Option 2: The children enjoyed very much at the Dussehra mela. — Incorrect. “Enjoy” needs an object. When there is no other object, a reflexive pronoun is used. Here that is “themselves”.
Corrected: The children enjoyed themselves very much at the Dussehra mela.

Option 3: The woman whom sold us these bangles gave us a discount. — Incorrect. The given sentence says “the woman whom sold”. This is wrong: the woman did the selling, so the subject form “who” is needed.
Corrected: The woman who sold us these bangles gave us a discount.