Solenocytes also known as flame cells are excretory structures found in Platyhelminthes. They are meant for excretion and osmoregulation. The solenocyte carries a flagellum, which wafts excretory products along the tubule.
Hence, option 'A' is correct.

Explanation:
Solenocytes are specialized excretory structures found in Platyhelminthes, which are commonly known as flatworms. These structures are responsible for excretion, osmoregulation, and maintenance of water balance in the organism.

Platyhelminthes:
Platyhelminthes is a phylum of invertebrate animals that includes flatworms. They are characterized by their flattened body shape and bilateral symmetry. Platyhelminthes are a diverse group of organisms that inhabit various environments, including freshwater, marine, and terrestrial habitats.

Solenocytes:
Solenocytes are flame cells or protonephridia found in the excretory system of Platyhelminthes. They are so named because of their flame-like appearance due to the beating of cilia on their surface. Solenocytes are primarily responsible for excretion, filtration, and maintenance of osmotic balance.

Excretory function:
The solenocytes in Platyhelminthes help in the removal of metabolic waste products, such as ammonia, from the organism's body. They filter the body fluids and collect waste materials, which are then excreted out of the body through specialized excretory ducts.

Osmoregulation and water balance:
Solenocytes also play a crucial role in osmoregulation and maintaining water balance in Platyhelminthes. They help regulate the concentration of solutes and water in the body fluids, ensuring that the organism maintains a stable internal environment.

Other excretory structures:
While solenocytes are the main excretory structures in Platyhelminthes, other organisms belonging to different phyla have different excretory structures. For example:
- Annelids (phylum Annelida) have nephridia as their main excretory structures.
- Molluscs (phylum Mollusca) have nephridia or kidneys for excretion.
- Echinoderms (phylum Echinodermata) have a water vascular system that helps in excretion and osmoregulation.

Therefore, based on the given options, solenocytes are specifically found in Platyhelminthes, making option A the correct answer.