Which of the following is an example of a bound morpheme?

A bound morpheme is a type of morpheme that cannot stand alone as a word, meaning it must attach to another morpheme to convey meaning. In this case, "un-" serves as a prefix that modifies the meaning of the base word to which it is attached. For instance, when combined with the word "happy," it creates "unhappy," indicating a negation of the original meaning.

In contrast, the other options represent free morphemes, which can stand alone as independent words. "Cat" and "run" are complete words that convey meaning on their own, while "happy" also functions independently as a word with its own meaning. Thus, "un-" is a prime example of a bound morpheme due to its inability to exist by itself in a meaningful context.