Nephridium is the excretory organ found in earthworms.

Planaria, a type of flatworm, uses flame cells for excretion, not nephridia.

Tapeworms are also flatworms and use flame cells.

Roundworms have a different excretory system, often involving specialized glands, not nephridia. states that annelids (the group containing earthworms) have nephridia, while flatworms have flame cells.

So the correct answer is B) earthworm.

Textbook references:

• Idodo-Umeh College Biology, p. 188
• Extensive Biology for Senior Secondary Schools, pp. 378–379
• Essential Biology for Senior Secondary Schools, pp. 82–83, 307