Which of the following is an example of dissimilation?

Dissimilation is a phonetic process in which two similar sounds in a word become less similar to each other. This process often occurs to make pronunciation easier or more distinct. In the example where "handbag" becomes "hambag," the "n" sound is altered to an "m" sound because it is adjacent to another bilabial sound, the "b." This change helps to avoid the repetition of similar sounds, thus exemplifying dissimilation.

The other choices do not illustrate dissimilation effectively. "Film" becoming "fillum" demonstrates the addition of an extra vowel sound, a phenomenon known as epenthesis rather than dissimilation. "Artful" turning into "crafty" is an example of semantic change, where the meaning shifts significantly rather than focusing on phonetic alteration. Lastly, "beautiful" becoming "prettified" shows a morphological change, where a new verb form is derived, rather than reflecting the phonetic changes characteristic of dissimilation.