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

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. 

Answer 2

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