The correct answer is <em>"as soon as he comes in."</em>
Adverb clauses are chains of words that work together as an adverb. They can modify a verb, an adjective, another adverb, or a phrase. They give extra information, such as when, why, where and how. They possess a subject and a predicate, and start with a subordinating conjunction like <em>as</em><em>, </em><em>after</em><em>, </em>and <em>unless</em><em>. </em>
In this example, the adverb clause modifies the verb <em>call</em>.
