Which formula best describes the word "friendly"?

The formula that best describes the word "friendly" is the one indicating that an adjective plus the suffix "ly" transforms it into another form, which in this case pertains to how "friendly" is derived. Initially, "friend" is a noun. When the "ly" suffix is added, it describes a characteristic related to the noun, thus transforming "friend" into "friendly," which functions as an adjective describing someone or something that is kind or considerate towards others.

In understanding why this is the best choice, it emphasizes the relationship between the base noun and the resulting adjective when the suffix is applied. This process of modification from a noun to an adjective is a key aspect of word formation in English.