Final answer:
The progressive ratio schedule of reinforcement is more effective than the variable ratio schedule in indicating an animal's level of motivation for a reward, as it measures when the animal stops trying to obtain the reward.
Step-by-step explanation:
To determine which reinforcement schedule most effectively indicates the level of motivation of an animal to receive a reward, we compare the variable ratio and the progressive ratio schedules of reinforcement. The variable ratio reinforcement schedule involves providing rewards after an unpredictable number of responses, which leads to a high and steady response rate due to its unpredictability. This is exemplified by how slot machines operate, rewarding gamblers based on a variable ratio schedule. Conversely, a progressive ratio schedule involves increasing the response requirement for each subsequent reward, which can measure an animal's motivation by determining how hard an animal is willing to work as the task becomes progressively more challenging.
In terms of evaluating motivation, the progressive ratio schedule is more effective, as it specifically measures the point at which an animal stops trying to receive the reward, thus providing a clear index of an animal's level of motivation. Therefore, the answer to the student's question is: b) Progressive ratio.