Which of the following is a subordinating conjunction?

A subordinating conjunction is a word that connects a dependent clause to an independent clause, indicating a relationship between the two. The word "although" serves this purpose by introducing a dependent clause that expresses a contrast or concession. For example, in the sentence "Although it was raining, we went for a walk," "although" links the dependent clause "Although it was raining" with the independent clause "we went for a walk," showing how the first clause affects the interpretation of the second.

The other options list words that do not function as subordinating conjunctions. "Therefore" and "nonetheless" are transitional words that indicate a relationship between independent clauses, but they do not create dependent clauses. "Very" is an adverb that modifies adjectives or other adverbs, thus having no role as a conjunction. Therefore, "although" is the correct choice, as it effectively serves to create a connection between dependent and independent clauses.