Final answer:
1) Neptune is the only planet listed that has not been visited by a spacecraft. All other planets mentioned have been explored by probes, and Earth has been extensively studied as we inhabit it.
Step-by-step explanation:
The only planet that has not been visited and studied using instruments aboard a spacecraft is 1) Neptune. Each of the other options provided has indeed been studied by spacecraft. Mercury has been visited by the MESSENGER spacecraft, Saturn by the Cassini probe, and even Pluto with the New Horizons mission, while we obviously inhabit and have extensively studied Earth. Neptune is the eighth and farthest known planet from the Sun in our solar system and has been observed with telescopes but not yet explored up close via spacecraft. Exploring these celestial bodies helps us understand more about the potential habitability of other worlds and the evolution of our own planet. The largest trans-Neptunian objects (TNOs), which include Pluto and Eris, are classified as dwarf planets. Our solar system probe missions have also landed on or observed various moons, asteroids, and comets, contributing to our understanding of the solar system's composition and history.