Final answer:
Some combinations of two, three, or four letters that stand in for a vowel sound are 'ea', 'oo', 'oa', 'ei', 'oi', and 'ou'.
Step-by-step explanation:
The letters a, e, i, and o are always vowels. However, when it comes to combinations of two, three, or four letters that stand in for a vowel sound, we can have different options. In the given examples, we have the following combinations:
- ea in 'head'
- oo in 'hook' and 'book'
- oa in 'boat'
- ei in 'weigh'
- oi in 'soil'
- ou in 'bout'
These combinations represent different vowel sounds in English.