The correct answer is D. Transfer of pollen from anther to stigma.
Pollination is the transfer of pollen grains from the male anther of a flower to the female stigma of the same or another flower. This process is necessary for sexual reproduction in flowering plants.
Option A is incorrect because the growth of the pollen tube in the ovule is a later stage in the process of pollination.
Option B is incorrect because visiting flowers by insects is not the same as pollination. Insects can help to pollinate flowers by carrying pollen from one flower to another, but they are not essential for pollination to occur.
Option C is incorrect because germination of pollen grains is a process that occurs after pollination. Germination is the process by which a pollen grain starts to grow into a pollen tube.
I hope this helps!