A(n)
makes clauses dependent.
adjective
adverb
coordinating conjunction
subordinating conjunction

A Subordinating conjunction (Option D) makes a clause dependent.

Explanation:

A sentence can have two clauses: Independent or the main clause and the dependent clause.

The Independent clause is a complete thought in itself. It has a subject and a predicate (with verb). But a dependent clause is dependent on the main clause to complete its meaning.  These two clauses are joined using subordinating conjunction. Example of subordinating conjunctions are although, because, whenever, before,while, when. Use of these words make the clause dependent on the main clause.

A coordinating conjunction might join two independent clauses also, so the correct answer Option D.

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