When you push on an object such as a wrench, a steel pry bar, or even the outer edge of a door, you produce a torque equal to the force applied times the lever arm. at what angle to the lever arm should a force be applied to produce maximum torque and why?

Since youc oncetrate all your force directly towards the moment arm it means that you push  it at an angle  of your force is directed to the left or the right and I bet that it must be 90 degrees to the bar. Obviuosly, if you are about to push it you will do it  straight up but not in a zig zag way. In other words, it should be perpendicular to the arm because the torque can be produced only if force is applied at a constant index (90).
Hope that helps! Regards.

Liquid. good luck : )