0.5 molal aqueous solution of a weak acid (HX) is $20 \%$ ionised. If $K_f$ for water is $1.86 K\; kg\; mol^{-1}$, the lowering in freezing point of the solution is :

Question

( A ) 1.12 K
( B ) -0.56 K
( C ) -1.12 K
( D ) 0.56 K