Explanation:

- Fat cell (Option 1): These cells are involved in storing energy in the form of fat, found mainly in animals, including humans. They are not related to excretion or filtration processes.

- Flame cell (Option 2): These specialised cells are found in Platyhelminthes (flatworms). They act similarly to kidneys, removing waste by filtering fluids in the organism. Their name derives from the flickering movement of the cilia within the cell, resembling a flame.

- Stem cell (Option 3): These are undifferentiated cells with the ability to differentiate into various cell types. They are not directly involved in excretion.

- Sponge cell (Option 4): The term doesn't specifically relate to a known excretory or filtration function in Platyhelminthes but rather recalls sponges, a different animal group, which mainly features cells like choanocytes.