The missing character is R.
The other three characters, A, C, and O, are all vowels. R is a consonant.
Vowels are letters that make a sound when you say them. The five vowels in the English alphabet are A, E, I, O, and U.
Consonants are letters that make a sound when you say them, but they are not vowels. The other 21 letters in the English alphabet are consonants.
In the question, the four characters are arranged in a circle. The first character is A, the second character is C, the third character is O, and the fourth character is ?. The missing character must be a consonant, because the first three characters are all vowels. The missing character must also be different from the first three characters, because the first three characters are all different. The only consonant that is different from the first three characters is R. Therefore, the missing character is R.