The correct answer is option 3 - Earthworm.

In Earthworms, cross-fertilization occurs because they are hermaphrodites, which means they have both male and female reproductive organs. Earthworms have both testes and ovaries, allowing them to produce both sperm and eggs. During mating, two earthworms come together and exchange sperm. Each worm then uses the received sperm to fertilize its own eggs. This exchange of genetic material ensures cross-fertilization, as genetic material from two different individuals is mixed.

It`s important to note that option 1 (Hydra), option 2 (Ascaris), and option 4 (Silkworm) are incorrect because they do not engage in cross-fertilization. Hydra reproduces asexually through budding, Ascaris is dioecious (with separate male and female individuals), and Silkworms are also dioecious and undergo internal fertilization through copulation.