In which one of the following reactions does the heat change represent the heat of formation of water?

Question

$$\begin {array} {1 1} (a)\;(a)\;2H_2+O_2  \;\to\;2 H_2O; \Delta H=-116\; kcal \\ (b)\;(b)\;H_2+I/2O_2  \;\to\;2 H_2O; \Delta H=-58\; kcal \\ (c)\;(c)\;H^+ +OH^-  \;\to\; H_2O(l); \Delta H=-13.7\; kcal \\ (d)\;(d)\;C_2H_2+2 \frac{1}{2} O_2  \;\to\;2 CO_2+H_2O; \Delta H=-3\; kcal \end {array}$$