A machine is designed to lift an object with a weight of 12 newtons. If the input force for the machine is set
at 4 newtons, what is the mechanical advantage of the machine?

If the input force for the machine is set at 4 newtons, then the machine may or may not be able to lift the object.  If it IS able to lift an object that weighs 12 newtons, then the mechanical advantage of the machine must be at least  3  or more.

Mechanical advantage is calculated by output force/input force (Fo/Fi).So, 12/4=3.