Which phylum includes organisms with jointed legs and an exoskeleton?

The correct choice is Arthropoda, as this phylum is characterized by key features that include jointed legs and an exoskeleton. Organisms within this phylum, such as insects, arachnids, and crustaceans, possess a hard outer structure made of chitin, which protects them while providing support. The jointed appendages allow for greater mobility and adaptability in their environments, enabling these organisms to occupy various ecological niches.

In contrast, the other choices do not share these defining characteristics. The Chordata phylum includes animals with a notochord, such as mammals, fish, and birds, but does not encompass those with an exoskeleton or jointed legs. Mollusca, which comprises animals like snails and octopuses, typically have soft bodies, and while some have hard shells, they do not feature jointed legs. Lastly, Annelida includes segmented worms, which have bodies divided into segments but lack the jointed appendages and exoskeleton that define arthropods. This understanding highlights the unique attributes of the Arthropoda phylum and confirms why it is the correct answer.