Understanding Positive Reinforcement in Homework Submission

Operant conditioning is a learning process through which the strength of a behavior is modified by reinforcement or punishment. Consider the following scenario: A student named Emily receives points for completing homework assignments on time, which can later be exchanged for privileges such as free time or a preferred activity. This system encourages her to consistently submit assignments promptly.

Which of the following principles of operant conditioning best explains Emily's increased behavior of timely homework submission?