Length of the pollen tube depends on the distance between

Question

$\begin{array}{1 1} (a)\; \text{Pollen grain and upper surface of stigma} \\ (b)\;\text{Pollen grain on upper surface of stigma and ovule} \\ (c)\; \text{Upper surface of stigma and lower part of the style} \\ (d)\; \text{Pollen grain in the anther and upper surface of the stigma} \end{array}$