Q.Which of the following is the strongest acid among all ?

The hydrogen halides (HF, HCl, HBr, HI) can be made by direct combination (violent for F2, reversible/impure for Br2 and I2), by displacement of a halide salt with concentrated H2SO4 (works for HCl and HF, but HBr/HI are oxidised by the acid and so must instead be made by hydrolysing a phosphorus trihalide, PX3 + 3H2O -> H3PO3 + 3HX), or from H2S plus a halogen.

Bond dissociation enthalpy and percentage ionic character both fall steeply from HF (562 kJ/mol, 43%) to HI (299 kJ/mol, 7%), so thermal stability falls HF > HI > HBr > HCl is the observed decomposition-resistance order, while boiling point instead follows HCl < HBr < HI < HF — HF is anomalously high-boiling because of strong hydrogen bonding absent in the others. All four dissolve in water with extensive ionisation to give the hydrohalic acids; HCl, HBr and HI are strong acids, but HF is comparatively weak at low concentration, becoming much stronger at high concentration because free F- associates with un-ionised HF to form the bifluoride ion HF2- (a type of salt — NaHF2, KHF2, NH4HF2 — unique to fluorine), pulling the ionisation equilibrium forward; overall …