Which is the most curious letter?

“C”

While 'C' can represent curiosity (as in 'curious cat'), it doesn't play on the word 'why' or the question itself.

“Q”

'Q' may seem curious because it often requires a 'U' to follow, but it doesn't relate to the concept of questioning or wonder as 'Y' does.

“A”

'A' might be considered the beginning of the alphabet, but it doesn't convey the same sense of inquiry or curiosity that is inherent in the letter 'Y'.