It's easy! due the other options are out of logic and not congruent with the first statement.
If the action plan is already implemented that means it's already into effect, the plan is the solution to a problem that has been identified; and if the plan was put into effect, it's been already developed <em>(that's the solution)</em> and off course there's no way to evaluate any results before the plan has even been developed.
Thereby the correct is (A.), if the plan is implemented, the next steps are: <em>monitoring and evaluation</em>.
