What constitutes a complex sentence?

A complex sentence is defined as consisting of an independent clause, which can stand alone as a complete thought, and one or more dependent clauses, which cannot stand alone and are introduced by subordinating conjunctions (such as although, because, since, etc.). This structure allows for the incorporation of additional information or ideas that enhance the main idea presented in the independent clause.

For instance, in the sentence "Although it was raining, we went for a walk," "we went for a walk" serves as the independent clause, while "Although it was raining" is a dependent clause that adds context.

The other options describe different types of sentence constructions. Two independent clauses linked by a coordinating conjunction define a compound sentence, while a single independent clause describes a simple sentence. An independent clause paired with a noun clause, although structurally valid, does not capture the essential element of dependency that defines a complex sentence. Thus, the clear requirement for a complex sentence is the presence of at least one dependent clause accompanying an independent clause.