Final answer:
Parsecs, megaparsecs, and gigaparsecs are units used for very large cosmic distances, offering a practical way of representing the enormous scales in the universe beyond what is feasible with light-years.
Step-by-step explanation:
The units parsecs, megaparsecs, and gigaparsecs are necessary in addition to the light-year when studying distances in space because (d) they are convenient units for large cosmic distances. A parsec, which is equivalent to 3.26 light-years, was historically significant for astronomical measurements, particularly when distances were obtained from parallax measurements. However, as distances in astronomy can be extremely large, parsecs and its multiples, such as megaparsecs (one million parsecs) and gigaparsecs (one billion parsecs), provide more practical and manageable units for these vast scales. Megaparsecs and gigaparsecs are particularly useful when talking about distances between galaxies or the size of the universe, as using light-years would result in impractically large numbers.