After choosing the best solution, an individual must

define the problem
evaluate solutions
develop an action plan
implement the action plan

A is incorrect because the problem must have already been determined for there to have been a solution chosen. Likewise, evaluating the solution is no longer necessary if the best solution has already been determined. Implementing an action plan is not possible until one has been developed. (With the given information it should be assumed that an action plan has not been developed yet.) 

Therefore, the answer is "develop an action plan" 

The answer is a i believe