You should use a conjunction to introduce an adverbial clause.
a. coordinating
b. correlative
c. plural
d. subordinating

Answer : d. subordinating

A certain clause may qualify as an adverb clause as long as it has a subject and a verb, it must also has a subordinate conjunction that keeps the statement from expressing a single complete thought and lastly, the clause must answer the wh questions when, where, why and how. Examples of adverbial clauses are the following : 

Gemma cleaned the entire house until her whole body aches. The adverbial clause is "until her whole body aches.

I run to the door once I heard the chimes' sound.  The adverbial clause is " once I heard the chimes' sound"