Which one of the following environments will sound not travel through?
a. solid
b. gas
c. liquid
d. vacuum

Vacuum is the environment through which sound will not travel through. The correct option among all the options given in the question is option "D". It is a fact that sound requires a medium for travelling through and vacuum does not provide any kind of medium. The sound needs to vibrate against the medium for travelling, but in case of vacuum there is no medium to vibrate and hence the question of transmission does not arise.  In case of the other options mentioned in the question, sound will definitely travel.

Liquid. good luck : )