The current pattern is _RAIN and we have already guessed a, b, e, i, n, r, t. This rules out common options like "brain" (because b was guessed) and "train" (t was guessed), leaving possible words such as "drain" and "grain." I haven't guessed 'd' yet, and it would reveal the word if it is "drain." So I'll choose 'd' as my next guess.