I answered
i
It's a smart question actually... I didn't actually check but most of my chat messages start with "I" (like this answer).
Why the question is smart? "H" "I" "W" and "Y"... the H/W are question starters... H/I/Y are for subject pronouns, I'm pretty sure if there's was a statistic for this.. those would be the most used ones...
I think probably the letter "W"... just a guess.