In this question, the word that differs from the other three in the position of primary stress is:

**A. enough**

The primary stress pattern for each word is as follows:

- enough: e-**nough** (stress on the second syllable)
- carton: **car**-ton (stress on the first syllable)
- pepper: **pep**-per (stress on the first syllable)
- butter: **but**-ter (stress on the first syllable)

Therefore, "enough" has its primary stress on the second syllable, distinguishing it from the others.