This is a syllable or word element that can only occur in front of or after a root or stem

Affix.

An affix is a grammatical unit that is attached to a word's stem to form a new word. They are important in English because they are one of the main ways in which the language creates new words.

The two most common ones are: prefixes, which are placed before the stem (ex. pre-, in-, un-) and suffixes, which are placed after the stem (ex. -ly, -ness).

your answer is c) experiments with water vapor showed that elements combine in specific proportions.