Final answer:
The conditioned stimulus in the given scenario where a firefighter associates the smell of smoke with fear is a) The smell of smoke from a barbeque, as it is a neutral stimulus that becomes associated with fear after repeated experiences of dangerous fire situations.
Step-by-step explanation:
In classical conditioning, a conditioned stimulus (CS) is a previously neutral stimulus that, after becoming associated with the unconditioned stimulus (US), comes to trigger a conditioned response (CR). In the scenario where a firefighter associates the smell of smoke with fear, the conditioned stimulus is a) The smell of smoke from a barbeque. This is because the smell of smoke was originally a neutral stimulus that has become associated with the dangerous and fear-inducing situation of entering a burning building, which is the unconditioned stimulus that elicits the unconditioned response (fear and increased heart rate). Thus, when later exposed to the smell of smoke, even in a harmless context such as a barbeque, it triggers the conditioned response of fear that was learned through the repeated pairing of smoke (CS) with the dangerous situation (US).