If you can complete more than 20 repetitions over three sets with your current weight, you should a. increase the weight until no more than 20 repetitions and three sets are possible b. decrease the weight and increase the number of sets to 8 to 10 c. increase the weight until you can do no more than 2 to 4 repetitions in three sets d. keep the weight the same and increase the number of repetitions and sets until you are tired

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.