Question

the tails on choanocytes are called flagella collar cells plankton detritus

Explanation:

Choanocytes (collar cells) have a tail - like structure called a flagellum (plural: flagella). Collar cells are the cells themselves, plankton is a type of organism, and detritus is organic debris, so they don't match the description of the tails on choanocytes.

Answer:

flagella