A plucked guitar string would vibrate for a longer time if the guitar had no sounding board.
Work Step by Step
A guitar relies on a sound board, which is a surface that the strings vibrate against, and set into motion. The sound is made louder because the the vibrating sound board has a large surface, and moves a lot more air than the vibrating strings can move on their own. If the sounding board were absent, less air would be moved per unit of time, which means that the energy of the vibrating string would be transformed into sound waves more slowly, i.e., it would vibrate for a longer time, at a lower volume.