You are pushing on a heavy door, trying to slide it open. if your friend stands behind you and you push, how have the forces changed?
a: the applied force does not change
b: there will be less friction
c: the net force will decrease
d: the net force applied will increase

I would be D, The Net Force Applied Will Increase. 

Because, B would be out of the question if it's re-guarding the person's friction. Because the more you lean and push the more friction you have within your hands and feet. 

Net Force is when Force is acting on a object by another force such as their hands. So if it decreases then there would be no one to open it. But if the Net Force increases then their is more force to apply it open ! 

Please let me know if i am correct !