I'd say that the aspect of a problem-and-solution essay that refers to the process of putting the solution into effect is implementation. The correct answer is A.
Explanation:
The answer is implementation because it means exactly to put into action a statement. To put an idea into effect. It means to execute. In an essay of this kind, the idea is executed in order to demonstrate the solution, in order to go through the problem and the way to solve it by executing the initial statement.
