A. increase the weight until no more than 20 repetitions and three sets are possible
the concept behind this is called Progressive Overload: the human body and the muscular tissue, in particular, must be challenged with an increasing intensity, otherwise, it becomes accustomed to what it is doing. So, if you can complete more than 20 repetitions on three sets, you should increase the weight until no more than 20 repetitions and three sets are possible, increasing in doing so the difficulty of the exercise.
