Which of the following best describes a digraph?

A digraph is best described as a pair of letters that make a single sound. This definition captures the essence of how digraphs function in phonetics; they consist of two letters that together create one phonetic sound. For example, in the word "chop," the letters "ch" function together to produce one sound, rather than two distinct sounds for each letter.

The other options do not accurately define a digraph. A single letter representing a sound refers to a phoneme, while a group of letters that forms a consonant blend involves multiple sounds that can be heard distinctly, such as in "st" or "bl." A sequence of three or more letters could refer to many other linguistic structures, such as trigraphs or other letter combinations, not necessarily indicating a single sound. Therefore, the emphasis on the fact that a digraph specifically consists of two letters creating one sound is what makes the third option the most accurate description.