The correct answer is c) gills.
Explanation: Aquatic arthropods, such as crabs and shrimp, possess gills, which are specialized structures that allow them to extract oxygen from water. These gills are crucial for their survival in an aquatic environment. Conversely, terrestrial arthropods, such as spiders and insects, have adapted to life on land and do not possess gills. Instead, they often rely on tracheae and book lungs for respiration. The other options listed—antennae, chelicerae, and pedipalps—are structures that can be found in both aquatic and terrestrial arthropods, serving various sensory and feeding functions. Therefore, gills are unique to aquatic forms among the options provided.