The correct answer is option 1: three pairs of legs. This means that insects have a total of six legs. When we observe insects such as ants, bees, or butterflies, we can count three sets of legs attached to their thorax. This distinguishes insects from organisms with different numbers of legs.

Option 2, four pairs of legs, is incorrect, as insects only have three pairs. Option 3, consisting of the head, thorax, and abdomen, is a general description of an insect`s body structure, but it does not specifically define what sets insects apart from other organisms. Option 4, a pair of wings and antenna, describes additional features that insects commonly possess, but they are not the defining characteristic of an insect.

Thus, the correct answer remains as option 1.