Thoreau is saying that it makes no sense for a simple man, who had nothing to do with the formation of such a beautiful pond, to name the pond. Giving the pond such a simple name as "Flint's Pond" is discrediting all of the beautiful animals and beings who dwell there such as the ducks, the fish, and the beautiful flowers. The pond was named in the selfish thought of money, rather than majesty.
The correct answer is phrase. A phrase is a group of words that work together to express an idea and can function as a noun, adjective, verb, or adverb.
