When water and nutrients are available, algae grow and multiply rapidly by the process of _________.

The correct answer is fragmentation.

When sufficient water and nutrients are available, algae (such as Spirogyra) rapidly grow and multiply through a mode of asexual reproduction called fragmentation.

In this process, an alga breaks up into two or more smaller pieces or fragments upon maturation. Each fragment subsequently grows into a new individual alga, allowing them to cover a large area of a water body in a very short period.

Let us briefly look at why the other options are incorrect:
- Spore formation: This is a method of reproduction commonly seen in fungi and non-flowering plants like ferns and mosses under unfavorable conditions.
- Pollination: This is the sexual reproduction process in flowering plants where pollen grains are transferred from the anther to the stigma.
- Budding: This process is characteristic of organisms like yeast and hydra, where a new organism develops from an outgrowth or bud due to cell division at one particular site.