Which of the following best describes the term "prefix"?

The term "prefix" is accurately described as a word part that is added to the beginning of a base word or root word. Prefixes are used in the English language to modify the meaning of the root words they precede, often providing additional context about time, location, negation, or other attributes. For example, the prefix "un-" can be added to the word "happy" to form "unhappy," changing the meaning to its opposite.

In contrast, the other options do not align with the definition of a prefix. A word part added to the end refers to a suffix, while standalone words do not fit the description of a prefix at all. Additionally, a group of letters without meaning does not represent a prefix since prefixes have significant meanings and contribute to the understanding of the words they modify. Therefore, option B accurately encapsulates what a prefix is in the context of word formation.