The correct answer is option A, that is, blue crab.
A food web comprises all the food chains in a single ecosystem. Each living thing in an ecosystem is a component of various food chains. Each food chain is one probable path, which nutrients and energy may take as they move via the ecosystem.
The species in food webs are divided into classifications known as trophic levels. These levels are differentiated into producers, consumers, and decomposers.
The tertiary consumers refer to the species, which consume the secondary consumers. They are at the fourth trophic level. In the marine food web, the blue crab can hold the position of tertiary consumers.