Which of the following best describes a vowel digraph?

A vowel digraph is defined as two vowels that are placed together in a word to create a single sound. In options representing this concept, the correct description indicates that when these two vowels are combined, typically one vowel sound is prevalent, often the first vowel. This aligns with common examples of vowel digraphs in English like "ea" in "bread" or "ai" in "rain," where the combined vowels produce a singular sound rather than their individual vowel sounds being pronounced distinctly.

In contrast, other options provide descriptions that do not accurately capture the essence of a vowel digraph. The first option suggests both vowels are sounded, which is more aligned with a diphthong rather than a digraph. The third option describes separate vowel sounds which is characteristic of a different linguistic phenomenon. The fourth option refers to a single vowel sound in a syllable, which does not pertain to the combination of two vowels. Thus, the best description of a vowel digraph correlates with the idea that the two vowels create just one sound, emphasizing the predominant sound of the first vowel in many instances.