Question

Expand the underlined phrase into a clause in the sentence given below:
While watching a horror movie, Sameer heard a loud knock on his bedroom door.

• A. NO CHANGE
• B. After watching a horror movie
• C. Before watching a horror movie
• D. While he watched a horror movie

Answer 1

The correct option is D While he watched a horror movie

An adverb clause is a group of words containing a subject and a verb, and it functions as an adverb, by modifying an adjective, verb or an adverb. Adverb clauses generally begin with a subordinate conjunction (since, because, while, etc.)
Generally, while expanding an adverb phrase to a clause, the subject is added and the present participle (verb + ing) is changed into a verb. (If there is an auxiliary/helping verb required, it is added.)
The underlined words form a phrase, it has no subject. Hence the sentence needs to be changed and option A is incorrect.
Option D is correct because the subject "he" is added and the verb "watched" replaces the present participle "watching".
Option B is incorrect because it's still a phrase.
Similarly, option C is incorrect because it is still a phrase.
Note: An adverb phrase may or may not contain an adverb, as long as it functions as one.