The excretory organs of Mollusca are
(a) Metanephridia
(b) Nephridia
(c) Green glands
(d) Protonephridia

Hint: The excretory organs of Mollusca are also known as ‘bojanus organs.’ This type of excretory gland is also seen in many invertebrates like annelids and arthropods. It typically comprises a ciliated funnel that opens into the coelom.

Complete answer:
The excretory organs of Mollusca are metanephridia. A molluscan body is found to be unsegmented. However, they do have a distinct head, a muscular foot, and a visceral hump. A soft and spongy layer of skin covers the visceral hump to form a mantle.
There is a space present between the hump and the mantle. This is called the mantle cavity. The Mantle cavity houses feather-like gills. These gills serve both respiratory and excretory functions. They are typical ‘metanephridia’ and are called bojanus glands. Excretory glands that function similar to metanephridia are called ‘saccate metanephridia’.

Additional Information: - The second-largest phylum under the kingdom of Animalia is Mollusca.
- They can be terrestrial and aquatic in their lifestyle. When aquatic, molluscs can be of two types, marine or freshwater.
- They have an organ system level of organization.
- Molluscs are bilaterally symmetrical, triploblastic. They are coelomate animals.
-Bilaterally symmetrical: When an imaginary line is drawn through the midline called the sagittal plane of an organism's body, it divides the body into two halves. If both these halves are mirror images of each other, then such an organism is said to be bilaterally symmetrical.
-Triploblastic. An organism that develops from all the three germ layers namely endoderm, ectoderm, and mesoderm.
- Coelomate: Animals possessing a body cavity called coelom.
- Molluscan’s body is covered in a calcareous shell.
- The anterior part of the head region possesses sensory tentacles.
- The mouth contains a file-like rasping organ. This is called radula and its function is feeding.
So, the correct answer is, ‘Metanephridia.’

Note: -‘Nephridia’ are a pair of organs found in invertebrates. Nephridium to invertebrates is what a kidney is to vertebrates. A nephridium may be of two basic types namely protonephridia and metanephridia.
-‘Protonephridium’ is a structure consisting of a network of dead-end tubules, lacking any internal openings. It is seen in Phylum Platyhelminthes, Rotifera, Nemertea, and Chordata.
- Saccate metanephridia are exclusively seen in arthropods, in the form of coxal glands in arachnids, and antennal or ‘green glands’ and maxillary glands in crustaceans.