Let us say you are given a problem statement and then you have suggested a solution. Then you have to justify your answer with respect to its effectiveness. You may say the following.
The solution will be effective if it is implemented properly and sincerely. It is a solution that is proposed with known data and knowledge. If the true situation is just the same in reality, then it will be effective.
To be effective, the solution must tackle the main point and root cause of the problem. I say, I have identified and tackled the root cause. I have considered some limitations too. If adequate funds, and resources are available then the solution will be effective.
Effective means, the problem does not repeat again and we do not have to solve it again. So I will say, I have given a solution that reduces the chance that the problem occurs again.
To be effective, the solution must be implementable and practical in reality. I will justify that my solution can really be done and how.