A subordinating conjunction usually introduces a(n) _____.

adjective clause
adverb clause
noun clause
prepositional phrase

Here's a chart listing the most common subordinating conjunctions ... FYI.

http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

Let me know what you think about how this type of clause is used.
http://grammar.ccc.commnet.edu/grammar/clauses.htm

Adverb Clause

A subordinating conjunction usually introduces an adverb clause.

To determine the answer to this question, we need to understand what a subordinating conjunction is and how it functions in a sentence.

A subordinating conjunction is a type of conjunction that is used to join two clauses together, with one clause being dependent on the other. When a subordinating conjunction is used, it creates a relationship of subordination between the two clauses, indicating that the second clause is subordinate to or less important than the first clause.

Now, let's look at the options provided:

1. Adjective clause: An adjective clause is a dependent clause that functions as an adjective, modifying a noun or pronoun. It provides more information about the noun or pronoun it modifies. While subordinating conjunctions can introduce adjective clauses, they can also be introduced by relative pronouns such as who, whom, whose, which, and that. So, a subordinating conjunction is not the only way to introduce an adjective clause.

2. Adverb clause: An adverb clause is a dependent clause that functions as an adverb, modifying a verb, adjective, or another adverb. It provides additional information about the action or situation described in the main clause. Subordinating conjunctions are commonly used to introduce adverb clauses. They indicate the relationship between the main clause and the adverb clause, such as cause and effect, time, condition, concession, etc. Therefore, a subordinating conjunction usually introduces an adverb clause.

3. Noun clause: A noun clause is a dependent clause that functions as a noun within a sentence. It can act as the subject, direct object, complement, or object of a preposition. Noun clauses can be introduced by subordinating conjunctions. These subordinating conjunctions signal that the clause is acting as a noun within the sentence. So, a subordinating conjunction can introduce a noun clause.

4. Prepositional phrase: A prepositional phrase is a group of words that begins with a preposition and ends with a noun or pronoun, which is called the object of the preposition. Although subordinating conjunctions and prepositions can both introduce dependent clauses, prepositions introduce prepositional phrases, not clauses.

Based on the explanations above, we can conclude that a subordinating conjunction usually introduces an adverb clause, but it can also introduce a noun clause.