Final answer:
The adult wearing a helmet while cycling is demonstrating the 'perception of vulnerability to head injury from a bicycle accident,' which is a part of the health beliefs model. This model affects health-related behaviors by balancing perceived risks and benefits.
Step-by-step explanation:
The component of the health beliefs model being demonstrated by an adult wearing a helmet when riding a bicycle due to concern for potential head injury is the perception of vulnerability to head injury from a bicycle accident. This model includes various aspects, such as perceived susceptibility, perceived severity, perceived benefits, and perceived barriers, which combine to influence health-related behaviors.
In the given scenario, wearing a helmet is a preventive action taken due to the individual's recognition of the risk and severity of potential head injuries, corresponding to the perceived susceptibility and severity constructs within the model.