The correct answer is C. Thesis statement.
Step-by-step explanation
A thesis statement is the main idea/sentence of an argumentative text. This sentence establishes the position that the author exposes in his text, additionally, this idea must be reinforced using reliable, truthful, and verifiable arguments so that the readers have credibility on the exposed idea. According to the above, the sentence presented is a thesis statement because it establishes the author's position. After all, he states that the "music shell at Webster Square is an important part of Covington's identity" and he provides three reasons. According to the above, the correct answer is C.