This can be based on Freud’s Psychosexual development theory. At the age of 1 to 3 years old, the child goes through the anal stage which means that C is the answer to this question. It is during the anal stage that the child discovers that it is possible to control both bowel and bladder movements.
This explains why some children may find it hard to be toilet trained. They have to understand that there is a designated time and space in order to release their urine or their feces. The success of this stage usually depends on how the parents will help their children during this stage.