Insects differ from other arthropods primarily because they:

Insects are primarily distinguished from other arthropods by having six legs. This characteristic is one of the most defining features that sets insects apart, as all insects belong to the class Insecta, which is characterized by this specific leg count. Insects have a body plan that includes a head, thorax, and abdomen, but the presence of six legs is a unique feature not found in other groups of arthropods, such as arachnids, which typically have eight legs.

While it is true that insects have a two-part body (the thorax and abdomen) and undergo molting like other arthropods, the defining feature that distinguishes them in general comparisons is indeed the six legs. The option encompassing all of the above does not better capture what makes insects distinct on its own, since the six legs are a primary identification point in classification within the arthropod phylum.