Which situation demonstrates meeting the goal of efficiency?

I believe the answer is: when you do something with as little waste as possible

Maximum efficiency is achieved if you manged to obtain the maximum benefit while making the least amount of expense as possible. In managing your daily life, this could be achieved by making sure that your time is spent carefully so it give as little waste as possible.

answer; i do believe its (b) 12.5 years of age;

this is not a question