You should use a _______ conjunction to introduce an adverbial clause.
A. plural
B. subordinating
C. coordinating
D. correlative
http://www.english-grammar-revolution.com/adverb-clauses.html
To determine the correct answer, we need to understand what an adverbial clause is and how it is introduced by a conjunction.
An adverbial clause is a type of dependent clause that functions as an adverb within a sentence, providing information about time, place, manner, condition, reason, or purpose.
In order to introduce an adverbial clause, we need a conjunction that can connect the dependent clause to the main clause. This type of conjunction is called a subordinating conjunction.
Therefore, the correct answer is B. subordinating.