Final answer:
Option b is correct option. Chelicerates have six pairs of appendages that include a pair of pedipalps, four pairs of walking legs, and a pair of chelicerae, which are specialized mouthparts used for feeding and venom injection in some species.
Step-by-step explanation:
The chelicerates are a group of arthropods that includes spiders, scorpions, ticks, and mites. This group is known for having six pairs of appendages on their body.
These appendages include one pair of chelicerae, which are specialized mouthparts used for feeding and, in some species like spiders, for venom injection. Chelicerates also have a pair of pedipalps used for sensing the environment and manipulating food, and four pairs of walking legs for locomotion.
Unlike some other arthropods, chelicerates do not possess antennae or mandibles, and they do not have the ovipositors that some insects use for laying eggs. Based on the information provided, the correct answer to fill in the blank is 'b. Chelicerae'.
Therefore, the six pairs of appendages that chelicerates have include a pair of pedipalps, four pairs of walking legs, and a pair of chelicerae.