Pollination by water is called:
Water pollination occurs when pollen drifts on water in rivers, lakes, ponds, etc. and comes in contact with flowers directly, thereby allowing certain plants to reproduce. This process is called as hydrophily.Hydrophily works due to water currents that carry pollen either on rivers or lakes. These pollens float to another flower in order to germinate. This is a highly complicated method of reproduction which works for flowering plants of the same species. In order to achieve this, flowers need to make a lot of pollen to increase the chances of reproduction.
Hence the option iv) Hydrophily is the correct answer.