Consider the following reactions:(i) ${\text{H}}^{+}(aq)+{\text{OH}}^{-}(aq)={\text{H}}_2\text{O}(l)$, $\Delta H=-x_1{\text{ kJmol}}^{-1}$ (ii) ${\text{H}}_2(g)+\frac{1}{2}{\text{O}}_2(g)={\text{H}}_2\text{O}(l)$, $\Delta H=-x_2{\text{ kJmol}}^{-1}$ (iii) ${\text{CO}}_2(g)+{\text{H}}_2(g)=\text{CO}(g)+{\text{H}}_2\text{O}(l)$, $\Delta H=-x_3{\text{ kJmol}}^{-1}$ (iv) ${\text{C}}_2{\text{H}}_2(g)+\frac{5}{2}{\text{O}}_2(g)=2{\text{CO}}_2+{\text{H}}_2\text{O}(l)$, $\Delta H=-x_4{\text{ kJmol}}^{-1}$ The enthalpy of formation of ${\text{H}}_2\text{O}(l)$ is:

The enthalpy of formation ($\Delta H_f^{\circ }$) of a compound is the enthalpy change for the reaction in which 1 mole of the compound is formed from its constituent elements in their standard states.The reaction for the formation of ${\text{H}}_2\text{O}(l)$ is:
$${\text{H}}_2(g)+\frac{1}{2}{\text{O}}_2(g)\to {\text{H}}_2\text{O}(l)$$
This matches reaction (ii). Therefore, $\Delta H_f=-x_2{\text{ kJmol}}^{-1}$.(Note: The question asks for the value, usually magnitude or the specific variable).