Final answer:
A discriminative stimulus is the correct answer as it indicates to an organism when a specific response will be reinforced.
Step-by-step explanation:
The stimulus that indicates reinforcement is available is known as a discriminative stimulus. This is a specific environmental event or cue which signifies to an organism that a certain response will lead to the anticipated reinforcement. For example, in a scenario with a dog being trained to sit, the dog will eventually understand that the command to sit, when obeyed, will result in a treat. The command becomes a discriminative stimulus indicating that sitting will be reinforced, typically by a treat. Therefore, the correct answer to the student's question is c. a discriminative stimulus.
In the context of operant conditioning, this concept is crucial for an organism to distinguish between different environmental cues to know which behaviors are reinforced and which are not. This understanding strengthens the association between the discriminative stimulus and the behavior.