To find the correct definition of "undergrowth" in the context of the sentence, we would look for content clues that relate to the characteristics or components of undergrowth. In the provided text, the phrases “thick shrubs” and “fire removes thick shrubs and undergrowth” can serve as clues that indicate what undergrowth might consist of.
The clues suggest that undergrowth refers to the dense vegetation that can include shrubs and other low-growing plants beneath taller trees. Therefore, the set of content clues that best points to the definition of "undergrowth" in this context would be “thick shrubs” and the notion of being removed by fire. These clues imply that undergrowth is made up of various smaller or low-growing plants that grow in the area below the larger trees.