Final answer:
Most organisms with exoskeletons are small because exoskeletons do not grow with the organism. Organisms must shed and form a new exoskeleton during growth, and a thicker exoskeleton is needed to support increased weight, becoming impractical for larger sizes. Hence, exoskeletons limit growth, and larger organisms cannot support them.
Step-by-step explanation:
Most organisms with exoskeletons are relatively small because of the physical limitations imposed by their exoskeletons. The exoskeleton, composed of chitin and often calcium carbonate, does not grow with the organism. Instead, arthropods must shed their current exoskeleton and form a new one underneath to accommodate growth, a process called molting. As an organism with an exoskeleton grows, the necessity for a thicker exoskeleton to support increased weight causes a substantial increase in chitin thickness. Since the weight increases by a factor of eight with every doubling in size, the exoskeleton becomes too bulky and heavy to be practical for supporting larger body sizes. In contrast, endoskeletons, found in vertebrates, are more efficient at growing with the organism and providing support for increased mass because muscles are attached on the outside.
Therefore, the correct answer to the given question is (a) Exoskeletons limit growth; larger organisms cannot support them. An increased body size would significantly thicken the exoskeleton to support the weight, which is not a practical way to maintain mobility and efficiency for larger organisms.