What is the significant feature of the first periodic comet identified?

The significant feature of the first periodic comet identified, Halley's Comet, is that it appears every 76 years. This comet, named after Edmond Halley who calculated its orbit, was the first discovered comet known to return at regular intervals. The 76-year periodicity is what makes Halley's Comet particularly notable, as it can be expected to be visible from Earth approximately once in a lifetime for most people. This predictable recurrence set the stage for our understanding of other comets and their orbits in the solar system.

The other choices either refer to different periods or do not correspond to Halley's Comet. Different comets may have their own unique orbits and visibility patterns, but Halley’s is distinctly marked by its 76-year cycle, making this particular characteristic well-known and important in the study of astronomy.