What do we call groups of letters added to the end of a word to change its meaning?

The correct choice is "suffix" because a suffix is specifically a group of letters that are attached to the end of a word to modify its meaning or change its grammatical function. For instance, adding "ly" to the word "quick" transforms it into "quickly," altering the word from an adjective to an adverb.

This term distinctly applies to letter groups that are appended at the end of words, thereby enhancing or shifting their meanings, which is central to understanding how word formation and language structure operate. In contrast, a prefix is added at the beginning, a root serves as the base of the word, and inflection refers more broadly to variations in word forms to convey grammatical relationships rather than the addition of letters strictly at the end.