Fluorine reactes with $ KHSO_4$ to form HF and $ \underline X$. Which one of the following is $ \underline X$?

Question

$\begin {array} {1 1} (1)\;SO_3 & \quad (2)\;K_2SO_4 \\ (3)\;K_2S_2O_8 & \quad (4)\;H_2S \end {array}$