Which is a necessary element of every effective physical activity action plan?
a. stating when and how often you will do the steps
b. identifying role models who have achieved the goal
c. developing a plan b in case you do not succeed
d. evaluating your skills to execute the action plan

Evaluating your skills is a key component to a successful physical activity action plan. Assessing yourself through physical tests (e.g. weight and girth measurements) could give you an idea what exactly you want to happen in your training program. Aside from knowing your structure, you should also assess yourself if you are fit enough or you have certain medical conditions that would prohibit you from performing certain activities (e.g. high blood pressure, hypertension). 

In doing so, you not only ensure the success of your training action plan, but you also secured your safety before executing the activities you assigned for yourself.

The answer would be a