C. Cabo San Lucas, which is on the Baja Peninsula, is a fabulous place to visit.
Non-essential expression has nonessential elements such as word, phrase, or clause that are not important in the meaning of the sentence. Even if you eliminate that element, the meaning of the sentence won't change.
Just like the statement found in letter C. The nonessential element here is "which is on the Baja Peninsula". This phrase only adds more information to the subject. Even if you omit this phrase, the meaning is still the same.
A. The airplane circled once, and then came in for a landing.
B. The mosquito bit me, and I have a huge welt on my arm.
D. To win the game, we need a miracle.
(All these three options have no nonessential element. All the words and phrases used are essential in completing the thought of the sentence.)
