Which of the following is considered a portmanteau?

A portmanteau is a linguistic blend of words, where parts of two or more words are combined to create a new word that incorporates the meanings of the original words. In the case of "brunch," it is formed by combining elements of the words "breakfast" and "lunch." This new word effectively conveys the idea of a meal that is consumed during the late morning or early afternoon, typically between breakfast and lunch.

The other options do not qualify as portmanteaus. "Fax" is derived from "facsimile," but does not blend two separate words meaningfully. "Babysitter" is a compound word formed by simply putting together "baby" and "sitter," which retains the integrity of each word without blending them into a new, distinct meaning. "Saxophone" is derived from the name of its inventor, Adolphe Sax, and does not represent a combination of two or more words with blended meanings. Thus, "brunch" stands out as the only term that exemplifies the characteristics of a portmanteau.