Explanation:

Correct option 3: Entering

The sentence is:

"Entering the hall, the show had started."

Let's analyze it step by step:

The sentence begins with the participle phrase:

"Entering the hall,"

This phrase is meant to describe the subject of the main clause that follows. So, the subject performing the action of "entering" should immediately follow the comma.

But what follows is:

"the show had started."

This implies that the show was the one "entering the hall" — which is illogical.

The sentence should be:

"Entering the hall, I saw that the show had started."
OR
"When I entered the hall, the show had already started."

The grammatical error is due to a dangling participle — the participle "Entering" lacks a proper subject.