Final answer:
c. Mercury has a day close to two Earth days long, as its full day-night cycle from the perspective of an observer on the surface is about 176 Earth days.
Step-by-step explanation:
The terrestrial planet that has a day close to two Earth days long is c. Mercury. Mercury rotates on its axis once every 59 Earth days. However, due to its unique orbital characteristics, the time for the Sun to return to the same place in the sky of Mercury, effectively marking a full day-night cycle for an observer on the surface, is approximately 176 Earth days. This period is equal to two Mercury years.
None of the other options given, Venus, Mars, or Pluto (which is no longer classified as a terrestrial planet), have a day close to two Earth days in length.