The correct answer is Mostly the ending.
A sentence fragment is a group of words that looks like a sentence. But it is not a complete sentence because there is usually a subject or a verb missing. A fragment can nor stand on its own. An independent clause ( containing a subject and and a verb) is needed for a fragment to become a sentence.
In Mostly the ending there is no verb, so this is not a sentence. It can not stand on its own because it has no meaning separately. All of the other examples are sentences because they have both a subject (He, Many people, It) and verbs (wrote, is, like).