Question

Combine the following sentences into a complex sentence with a subordinate noun clause:
He left. It is a mystery. [Intermediate]

• A. Why he left is a mystery.
• B. He is the one who left.
• C. He mystically left.
• D. He left with a mystery.

Answer 1

The correct option is A Why he left is a mystery.

A Complex Sentence consists of one principal clause and any number of subordinate clauses.
A Noun Clause is a subordinate clause which does the work of a noun in a complex sentence.
In this case,
Option A is correct. The subordinate clause why he left acts as the subject of the verb is in the principal clause. A subject is always either a noun or a pronoun. Henceforth, why he left is a subordinate noun clause as it behaves like a noun.
Option B is incorrect as the sentences have been joined by using a subordinate adjective clause.
Options C and D are incorrect because they are simple sentences and not complex ones because they are devoid of any subordinating conjunctions.