Which type of sea mammals have a fine mesh sieve in their mouths and feed on plankton and krill?

Baleen whales are distinguished by their unique feeding mechanism that involves the presence of baleen plates in their mouths. These plates act as a fine mesh sieve, allowing them to filter small organisms such as plankton and krill from the water. When baleen whales take in a mouthful of water, they close their mouths and use their tongues to push the water out while trapping the food within the baleen. This filter-feeding system is highly efficient for consuming large amounts of tiny prey that are abundant in their marine environments.

In contrast, dolphins and killer whales are toothed whales that rely on echolocation and their sharp teeth to hunt larger fish and marine mammals, rather than filtering small organisms. Humpback whales, although they are baleen whales as well, are often more recognized for their dynamic feeding techniques, like bubble net feeding, rather than the general description of baleen whales, which collectively refers to those that use the sieve-like structure for feeding. Thus, the defining characteristic of the baleen plates makes this option the correct answer for the question regarding marine mammals that filter feed on plankton and krill.