Which principle of joint operations purpose is to concentrate the effects of combat power at the most advantageous place and time to produce decisive results?

The answer is mass. It is one of the principles of war which means to concentrate combat power at the decisive place and time. What it meant from the military perspective is that you bring together whatever forces are necessary to accomplish a desired result. An example would be moving forces from diverse locations to all take part in one set-up, such as the seaborne attack of Normandy in three locations with hundreds of ships coupled with airborne troops jumping behindhand enemy lines. The combination of the two achieved results far superior to either one, and if these pieces hadn't come together at the same time (bad weather during the channel crossing) or at the correct place (mistakes in the drop zone coordinates away from targets), the overall effect is weakened and could even lead to a non-victory: not necessarily a downfall but certainly a failure to accomplish the mission.