Question

what adaptation allows scallops to swim quickly?
a highly developed adductor muscle that opens and closes the shell to jet water
fins on their shells
their cilia-covered feet
their byssus threads

Explanation:

Scallops use a jet propulsion mechanism for quick swimming. Their large, strong adductor muscle rapidly opens and closes their shells, forcing water out to propel them forward. Fins on shells, cilia-covered feet, and byssus threads do not enable this quick swimming: cilia/feet are for slow movement, byssus threads attach to surfaces, and scallop shell "wings" are not swimming fins.

Answer:

A highly developed adductor muscle that opens and closes the shell to jet water