The odd one out is C, IO. The other options, AE, AI, EI, and OU, are all digraphs, which are two letters that represent a single sound. The letter combination IO does not represent a single sound in English. It can be used to represent two different sounds: the long “i” sound as in “ice” or the long “o” sound as in “go.”
A digraph is a pair of letters that represent a single sound. Some common digraphs in English include:
- ch: as in “chip”
- sh: as in “ship”
- th: as in “thin”
- wh: as in “white”
- ea: as in “eat”
- ai: as in “rain”
- ei: as in “eight”
- oi: as in “oil”
- ou: as in “out”
Digraphs are important because they allow us to represent a wider range of sounds than we could with just the 26 letters of the alphabet.