Which of the following is a type of mollusk?

The correct answer is a scallop, as it is classified as a bivalve mollusk. Mollusks are a diverse group of animals that typically have soft bodies and, in many cases, a hard shell. Scallops belong to the class Bivalvia, which includes other shellfish like clams and oysters. They are known for their fan-shaped shells and ability to swim by rapidly opening and closing their shells.

The other options should be considered in their biological classifications. A flounder is a type of fish, characterized by its flattened body and ability to camouflage with the ocean floor. An octopus is a cephalopod, another class within the mollusk phylum, distinctively known for its intelligence and ability to change color and texture. Lastly, a crab is a crustacean, which is part of the arthropod phylum, not a mollusk. Recognizing these classifications helps in understanding the diversity within the animal kingdom and the specific characteristics that define each group.