Choose the grammatically correct sentence:

The correct answer is Option (3) → At no time should visitors enter the operation theatre without the doctor’s permission.

“At no time” at the start of a sentence:

• “At no time” means “never”. It already makes the sentence negative.
• When a sentence starts with it, the helping verb comes before the subject: should visitors enter.

Why the Other Options are Incorrect?

Option 1: At no time visitors should enter the operation theatre without the doctor’s permission. — Incorrect. After “At no time” at the start, the helping verb “should” must come before the subject “visitors”.

Option 2: At no time should not visitors enter the operation theatre without the doctor’s permission. — Incorrect. “At no time” is already negative. The extra “not” is wrong.

Option 4: At no time visitors should not enter the operation theatre without the doctor’s permission. — Incorrect. It adds a second “not” to “At no time”, and the helping verb “should” does not come before the subject.