An affix is a morpheme that is added to a base word (or root) to change its meaning or grammatical function. Affixes can be categorized as prefixes (which are added to the beginning of a word) or suffixes (which are added to the end of a word).
From your provided options, the two correct statements about affixes are:
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It sometimes changes a base word’s part of speech. (For example, adding -ness to "happy" changes it from an adjective to a noun: happy → happiness.)
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It can come before or after a base word. (Prefixes come before the base word, while suffixes come after the base word.)
In the example of "unethical," "un-" is a prefix, "-al" is a suffix, and "ethic" is the base word. All of these elements (including the base) are morphemes.