The sentence appears to test parallelism. Before the comma, we have "remove stains...and improve tooth appearance" (which is grammatically correct) and after the comma, we have "for less use...and keeping costs down," which is incorrect. "Less use and lower costs" would be better; in any event, those two benefits of whiteners must be parallel, so (A) is incorrect.

Continue focusing on the second parallelism issue. (B) is wrong because "to be used less" and "to keep costs down" are not parallel. While in (C), "for lessening use" and "for keeping costs down" is technically parallel, neither phrase is idiomatic. It's rare that "for –ing" will be correct.

Choice (E) is wrong because it alters the meaning of the sentence. By placing the word "enough" before "tooth appearance," it refers to some amount of tooth appearance. In the original construction, "enough" is synonymous with "sufficiently," meaning that the whiteners improve appearance sufficiently to require less use of other techniques. Choice (D), then, must be correct.