Answer: D) be placed both before and after the nonessential element.
Explanation: When we use commas to separate a nonessential element (or subordinate clause) that is in the middle of a sentence, from the rest of the sentence (usually the independent clause), we have to put a comma before and another one after the nonessential element, for example: Mary, my boyfriend's sister, is very tall. So the correct answer is the corresponding to option D.
