Final answer:
A crab is an example of nektobenthos, a type of organism that resides on the seafloor but can also swim or move above it, unlike the sessile coral or sponge.
Step-by-step explanation:
An example of nektobenthos is c.) crab. Nektobenthic organisms are those that reside on the seafloor but also have the capability to swim or move actively above it. Crabs fit this definition as they can move across the seafloor and also swim. Sponges and corals are typically sessile, meaning they are fixed in one place and do not move about, thus they are not considered nektobenthic. Seagrass is a type of plant, so it also does not fall into the category of nektobenthos.