Here's why:
* Prefix: A prefix is added to the beginning of a word to change its meaning. There's no part of "peeks" that comes before the base word.
* Suffix: A suffix is added to the end of a word to change its meaning or grammatical function. "S" in "peeks" is a suffix, but it's not added to change the meaning, it's added to make the word plural.
Therefore, "peeks" itself is the complete word, with no prefix or suffix.