Explanation

Creating a filter: When we read the prompt, something is off in the basic subject/verb check. The phrase after the comma makes the sentence ungrammatical. Every phrase in a sentence must be an independent clause, a dependent clause, or a modifying phrase. In this sentence, the phrase after the comma is none of those. The conjunction "but" introduces that phrase as if a subject and verb are coming to make it an independent clause, but they don't appear.

Applying the filter: Choice (D) solves the problem in that manner, with a proper conjunction, subject and verb in the words, "although they augment," respectively. Another solution would be to say simply "but augment" and lose the comma before the "but," in which case the entire sentence would be one independent clause. The correct answer is (D).