Identifying the Conditioned Stimulus

In classical conditioning, the process of associating a neutral stimulus with an unconditioned stimulus results in a conditioned response. A common experimental setup involves a dog that salivates when it sees food, which is an unconditioned stimulus (US). If a bell (neutral stimulus) is rung each time before the dog receives food, eventually, the dog will start to salivate merely upon hearing the bell. This phenomenon shows how learning can occur through association.

Consider a researcher who observes that a child becomes anxious when they hear the sound of a particular ringtone, which had previously been associated with an unpleasant event. This situation illustrates classical conditioning with a human subject.

Based on this understanding, what would be considered the conditioned stimulus (CS) in this scenario?