I'm choosing 't' because it's one of the most common consonants in English. With a 6-letter word that has an 'i' in the fifth position, there's a good chance 't' appears. Common words with this pattern might include "action", "option", "saving", etc. I've already guessed vowels 'a' and 'e' without success, so focusing on high-frequency consonants is a logical next step.