The introduction of water hyacinth (Eichhornia crassipes) into Indian waters to reduce pollution is an example of:

alien species invasions

The correct answer is Option (3) - alien species invasions

Alien species invasions occur when non-native species are introduced, either intentionally or unintentionally, to a new ecosystem. In some cases, these alien species become invasive and have detrimental effects on the native species and ecosystem. For instance, the introduction of the Nile perch into Lake Victoria in east Africa resulted in the extinction of more than 200 species of cichlid fish that were unique to the lake's ecosystem.

Similarly, invasive weed species like carrot grass (Parthenium), Lantana, and water hyacinth (Eichhornia) pose environmental damage and threats to native species in various regions. These invasive plants can outcompete native vegetation, disrupt ecological balance, and reduce biodiversity.