Final answer:
The medium that allowed sound waves from the firing of the cannon to reach John was the air.
Step-by-step explanation:
When John heard the loud boom of the cannon firing a second later from a distance, the medium that allowed the sound waves to reach him was the air. Sound requires a medium to travel through, and in this scenario, it is the air between the cannon and John's ears. Sound waves generated by the cannon moved through the air, eventually reaching John and allowing him to perceive the sound of the cannon firing.