The correct answer is B. At the beginning of a word
Explanation:
In grammar, a prefix refers to a letter, set of letters or syllable that works as an affix and is placed before or a the beginning of a root word or basic unit. This type of affixes differs from suffixes because suffixes are always placed at the end of words and not at the beginning of them. Examples of common prefixes include "un-" that means not as in the word "uncover", "pre" that means before as in the word "pre-existing" or the prefix "co-" that means with as in the word "cooperation". Considering this, it can be concluded prefixes always occur at the beginning of a word.
