The current word __art_ suggests that it could be a word related to heart or art, and a common letter in many words is 'l' or 's'. However, since 's' is not present and 'l' could fit in the blank space to form a word like 'SMART' or 'DART' or 'PART', I'll choose 'l' as it seems to be the least tried letter among the common ones and could lead to a valid word.