When crystal learned to play the piano as a child, her dad would reward her with candy as she practiced. sometimes he gave her candy after 6 minutes of practice, then after another 15 minutes, and then again after another 10 minutes. what type of reinforcement schedule is described here?

The correct answer is variable interval schedule. It is because this is considered to be a type of operant conditioning reinforcement by which the reinforcement is likely to be given to the response after a specific time has passed by which the time is likely unpredictable as it changes or variable schedule.