I'm choosing 'o' because it's one of the most common vowels in English. Since 'e' has already been guessed and was incorrect, and we have only one 'a' in position 8, guessing another common vowel like 'o' is strategic. Common letter frequency statistics in English suggest that after 'e' and 'a', the letters 'o', 'i', and 'n' are most frequent. Since we have a 9-letter word with only one revealed vowel so far, there's a good chance it contains another vowel like 'o'.