Final answer:
The example of a digraph among the given choices is 'c. oi in oil', where 'oi' is a vowel digraph representing a single sound. Digraphs are two-letter combinations that make one sound, crucial for spelling and pronunciation in English.
Step-by-step explanation:
The question asks for an example of a digraph, which is a pair of letters that represent a single sound. The correct answer from the provided choices is 'c. oi in oil', as 'oi' represents a single vowel sound. The other options do not represent digraphs because 'bl' in 'black' contains two consonant sounds, and 'str' in 'street' is a consonant cluster, not a single sound. The option 'oa' in 'road' could be considered a vowel digraph, but the question does not present it as the correct choice.
Reviewing the rules of digraphs is important for understanding how they function in English spelling. For example, long can be spelled with various digraphs like in 'bread', in 'beet', and in 'key'. When practicing, words can be sorted or underlined based on their digraph content, as in exercises that help consolidate understanding of how these pairs of letters contribute to word pronunciation.