Which of the following is an example of a compound noun?

A compound noun is formed when two or more words are combined to create a single noun that has a specific meaning. In this case, "scarecrow" is the correct choice because it consists of two distinct words: "scare" and "crow," which together refer to a figure used to scare away birds from crops. The combination of these two words results in a noun with a unique meaning that is different from its individual components.

The other options do not fit the definition of compound nouns. For instance, "nameless" is an adjective that describes something without a name, while "impeached" is a verb that refers to the act of being formally charged with wrongdoing, and "phonograph" is a noun but consists of the root word "phone" and "graph," which is more aligned with a technical description rather than a straightforward compound noun. Therefore, "scarecrow" stands out as the true compound noun among the choices.