The more reactivity of Phosphorouspentachloride $(PCl_5)$ is due to

$\begin{array}{1 1}(a)\;\text{ equatorial bonds are slightly longer than the axial bonds}\\(b)\;\text{axial bonds are stronger than the equatorial bonds}\\(c)\;\text{ axial bonds are slightly longer than the equatorial bonds}\\(d)\;\text{ axial bonds are slightly stronger and slightly shorter than the equatorial bonds}\end{array}$

As the axial bond pairs suffer more repulsive interaction from the equatorial bond pairs, therefore axial bonds have been found to be slightly longer and hence slightly weaker than the equatorial bonds; which makes $PCl_5$ molecule more reactive.
Hence (c) is the correct answer.