I'm choosing 'o' because it's a common vowel that appears in many 5-letter words ending in "er" such as "lover", "power", "cover", and "voter". Since I've already guessed two vowels (a, e) and one consonant (r), adding another common vowel would help reveal the word structure. The pattern '___er' could fit words like "rover", "poker", or "tower", making 'o' a high-probability guess.