______ is aquatic plant showing insect pollination

The correct answer is Option (4) → Water hyacinth

Option (1) Hydrilla — Incorrect. Hydrilla grows in fresh water and is pollinated by water.

Option (2) Vallisneria — Incorrect. In Vallisneria the pollen grains float on the water surface and are carried by water currents to the female flowers.

Option (3) Zostera — Incorrect. This marine sea-grass releases its pollen inside the water, where it reaches the stigmas.

Option (4) Water hyacinth — Correct. Not every aquatic plant uses water for pollination. In water hyacinth and water lily the flowers rise above the water and are pollinated by insects or wind, as in most land plants.

NCERT Extract:
Pollination – “… Pollination by water is quite rare in flowering plants and is limited to about 30 genera, mostly monocotyledons. As against this, you would recall that water is a regular mode of transport for the male gametes among the lower plant groups such as algae, bryophytes and pteridophytes. It is believed, particularly for some bryophytes and pteridophytes, that their distribution is limited because of the need for water for the transport of male gametes and fertilisation. Some examples of water pollinated plants are Vallisneria and Hydrilla which grow in fresh water and several marine sea-grasses such as Zostera. Not all aquatic plants use water for pollination. In a majority of aquatic plants such as water hyacinth and water lily, the flowers emerge above the level of water and are pollinated by insects or wind as in most of the land plants.”