Question

A group of words that doesn't express a complete thought is called a ________.

topic sentence
sentence fragment
controlling idea
thesis sentence

Answer 1

It's a Sentence fragment. Hope I helped. ;)

Answer 2

A topic sentence is a sentence that expresses what a body paragraph is about.

A controlling idea helps the reader understand what you're going to talk about in your writing. It is pretty much your opinion on the topic.

A thesis statement is a sentence that expresses what the writing is going to be about.

So, the answer is sentence fragment.