Monoecious animals are those which contain both male and female reproductive organs together. (D) earthworm and leeches

Introduction:
Monoecious organisms are those that have both male and female reproductive organs within the same individual. They are also known as hermaphrodites. In this response, we will discuss whether leeches, earthworms, Nereis, or earthworms and leeches are monoecious.

Leeches:
- Leeches are hermaphroditic organisms, meaning they possess both male and female reproductive organs within the same individual.
- They have both testes and ovaries, allowing them to produce both eggs and sperm.
- Leeches reproduce sexually by exchanging sperm during mating, and then each individual can lay eggs.

Earthworms:
- Earthworms are also hermaphroditic organisms.
- They possess both male and female reproductive organs, known as testes and ovaries respectively.
- During reproduction, two earthworms align their bodies in opposite directions and exchange sperm.
- After fertilization, each earthworm can produce eggs in its own clitellum.

Nereis:
- Nereis is a genus of polychaete worms, commonly known as clam worms or sand worms.
- The members of this genus are dioecious, meaning they have separate male and female individuals.
- The male individuals produce sperm, while the female individuals produce eggs.
- Therefore, Nereis is not monoecious.

Conclusion:
- Among the options given, leeches and earthworms are monoecious organisms.
- They possess both male and female reproductive organs within the same individual, allowing them to produce both eggs and sperm.
- Nereis, on the other hand, is dioecious and has separate male and female individuals.
- Therefore, the correct answer is A. Leeches and B. Earthworms.