The current pattern is "___ter_" with positions 4, 5, and 6 revealed as 't', 'e', and 'r'. One common word that fits this pattern is "mystery" (M Y S T E R Y), where with the guessed letters (a, e, r, t) the revealed pattern would be ___TER_. Guessing 'y' would reveal two positions if the word is indeed "mystery", and it’s a letter we haven’t guessed yet. Thus, I'll choose 'y'.