What term describes a smaller set of antenna found on crustaceans?

The term that describes the smaller set of antennae found on crustaceans is "antennule." Unlike the larger primary antennae, antennules are typically smaller and serve important sensory functions. They are instrumental in detecting changes in the environment, such as chemical cues and movement in the water.

The other terms do not refer specifically to the smaller set of antennae in crustaceans. The flagellum is usually associated with the tail end of certain organisms or as a structure for movement, pereopods refer to the walking legs of crustaceans, and macropod generally pertains to a group of marsupials, which is unrelated to crustacean anatomy. Therefore, antennule is the accurate term to describe these smaller sensory appendages.