How would you account for the following $NF_3$ is an exothermic compounds but $NCl_3$ is not.

Question

$\begin{array}{1 1} \text{(a) The N-Cl bond is Weak but the bond of N-F is strong. Hence $NF_3$ is an endothermic compound} \\ \text{(b) The N-Cl bond is Strong but the bond of N-F is Weak . Hence $NF_3$ is an exothermic compound}\\ \text{(c) The N-Cl bond is Weak but the bond of N-F is strong. Hence $NF_3$ is an exothermic compound} \\(d)\;\text{The difference in size of N and Cl is small ,But the difference in size of N and F is large} \end{array} $