Which of the following attributes characterize arachnids?

Arachnids are characterized by having a body structure that includes a cephalothorax, which is a fused head and thorax, along with four pairs of legs and the absence of antennae. This morphology distinguishes arachnids from other arthropods, such as insects, which possess three pairs of legs and one pair of antennae, and crustaceans, which have two pairs of antennae. The unique structural features of arachnids allow them to effectively perform their ecological roles, such as predation and other behaviors relevant to their survival.

In contrast to the other options, which either describe other classes of arthropods or mischaracterize arachnid features, the correct choice accurately reflects the defining traits of arachnids. By focusing on the specific configurations of legs and facial structures, one can clearly see the essential differences that categorize arachnids within the animal kingdom.