When water is added to conc. $H_2SO_4$ it gets very hot. This happens because dilution of conc. $H_SO_4$ is ........

Question

$\begin{array}{1 1} \text{(a) an exothermic reaction } \\ \text{(b) an endothermic reaction } \\ \text{(c) photo chemical reaction } \\ \text{(d) hydrolytic reaction } \end{array} $