Soln:

The pollen grain is placed on the flower’s stigma. If the pollen grain and the stigma are compatible, the pollen grain germinates on the stigma, taking advantage of the moisture and nutrients present. The pollen tube, which contains two non-motile male gametes, is formed as a result of germination. The pollen tube descends through the style to the ovule, where the two male gametes are discharged. As a result, the distance between the upper surface of the stigma and the ovule determines the length of the pollen tube. Length of pollen tube ensures pollens reach the stigma to conduct pollination.

So, the answer is (d) upper surface of stigma and lower part of style.