Final answer:
The law of effect states that behaviors followed by satisfying consequences will likely be repeated, while those with unpleasant consequences will be discouraged; the best description is option 3.
Step-by-step explanation:
Edward Thorndike's law of effect is a fundamental theory within operant conditioning, a type of associative learning. According to the law of effect, behaviors that are followed by satisfying consequences are more likely to increase in frequency. Conversely, behaviors that result in unpleasant consequences are less likely to be repeated. The correct description of the law of effect is option 3, "A behavior followed by positive consequences is likely to increase in frequency." This principle is the foundation of B.F. Skinner's work on operant conditioning, where animals or humans learn to associate their behavior with the consequences that follow, leading to behavior modification.