Detailed Explanation:

Pollination is precisely defined as the transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower. This is the actual act of pollination, distinct from subsequent reproductive processes.

Germination of pollen grains and growth of the pollen tube occur after pollination is complete. Insect visitation is merely a mechanism (agent) of pollination, not the process itself.

Therefore, the transfer of pollen grain from anther to stigma is the correct definition, making Option 4 the correct answer.