Answer:
T
Only 1 letter
Explanation:
" T " is the silent letter in the word ' whistle '.
The letter “ t ” in these words is an acoustically “ explosive ” one, and to sound it after an “ s ” or an “ f ” both of which expend “considerable breath ” - " is especially difficult and obscure. " Consequently the “ t ” sound is assimilated into its surroundings and becomes silent.
