Which of the following sentences is correctly formed?

The correct answer is Option (3) → The last bus having left, the students walked back to the hostel.

An opening phrase with its own subject:

• An opening phrase can have its own subject when it is different from the subject of the main part of the sentence.
• Here the phrase has its own subject, “The last bus”, with “having left”. The main part has a different subject, “the students”.
• Without “The last bus”, the sentence would say that the students had left.

Why the Other Options are Incorrect?

Option 1: My grandfather goes always for a walk after dinner. — Incorrect. “Always” tells how often. It comes before the main verb, not after it.
Corrected: My grandfather always goes for a walk after dinner.

Option 2: The ceiling fan was making a noise, so Mehul switched off it. — Incorrect. The option says “switched off it”. When the object of “switch off” is a pronoun, the pronoun goes between the two words.
Corrected: The ceiling fan was making a noise, so Mehul switched it off.

Option 4: The shopkeeper did not give us no receipt for the shoes. — Incorrect. The option says “did not give us no receipt”. One negative word is enough in a sentence.
Corrected: The shopkeeper did not give us any receipt for the shoes.