Which of the following is an example of an arachnid?

An arachnid is a member of the class Arachnida, which is distinct from insects and other animal groups. The defining characteristics of arachnids include having eight legs (as opposed to the six legs of insects), two main body segments (the cephalothorax and abdomen), and the lack of antennae. Spiders, which fall under the arachnid category, exemplify these features perfectly, making them a clear representative of this class.

In contrast, butterflies belong to the order Lepidoptera within the class Insecta and have six legs and distinct wings. Insects, by definition, have a three-part body structure with six legs. Pigweed, on the other hand, is a type of plant, which does not belong to the animal kingdom at all. These distinctions underline why the spider is the only correct example of an arachnid in the provided options.