The correct answer is: Chameleons are lizards known for their ability to change colors.
The main idea is the point of the paragraph, the thought that is supported by the sentences. It is usually the first sentece but not always, so you need to be careful. To find it, the suggestion is to ask this question: What is being said about the person, thing, or idea?
In this case the author answers with: "Chameleons are tree-dwelling lizards known for their ability to change body color." That is the main idea, in the first sentece of the paragraph. The rest of the sentences support this statement as they give information about chameleons, their color and why and how they change it.
A main idea is defined as, "The most important or central though of a paragraph." In this passage, it talks about chameleons being able to change color. So the answer is Chameleons are lizards known for their ability to change colors.
