Among the options provided, the correct answer is option 1, which is leech. Leeches are sanguivorous organisms, meaning they feed on blood. They have specialized structures, such as their suction cups and razor-sharp jaws, which allow them to attach themselves to their hosts and feed on their blood.

Option 2, earthworm, is not a sanguivorous organism. Earthworms are detritivores, meaning they primarily consume decaying organic matter in soil.

Option 3, nereis, is also not a sanguivorous organism. Nereis is a type of marine worm that mainly feeds on algae, plankton, and small invertebrates.

Option 4, ascaris, is a parasitic worm that infects the intestines of humans and animals. While it does consume nutrients from its host, it does not specifically feed on blood like leeches do.

Therefore, based on the given options, the only sanguivorous organism is option 1, leech.