$xC + \frac{y}{2}{H_2} \to \mathop C\limits_x \,\mathop H\limits_y \,\,\,\,\,\,\,\,\Delta {H_f}$
We get, $\mathop {xC}\limits_{\Delta {H_C} = b} + \mathop {\frac{y}{2}\,{H_2}}\limits_{\Delta {H_C} = c} \to \mathop {\mathop C\limits_x \,\mathop H\limits_y }\limits_{\Delta {H_C} = a} $
Therefore, heat of formation
$\Delta H = \left( {xb + \frac{y}{2}c - a} \right)\,cal.$
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$B$ will be
Set $1$: $Al_2O_3 .xH_2O\, (s)$ and $OH^-(aq)$
Set $2$: $Al_2O_3 .xH_2O\, (s)$ and $H_2O\,(l)$
Set $3$: $Al_2O_3 .xH_2O\, (s)$ and $H^+(aq)$
Set $4$: $Al_2O_3 .xH_2O\, (s)$ and $NH_3(aq)$
(Atomic mass, $Ag =108, Br =80\, g\, mol ^{-1}$ )