Final answer:
The walk-in customer could reasonably believe that William has the authority to write an insurance contract because of his attire and office signage.
Step-by-step explanation:
The walk-in customer could reasonably believe that William has the authority to write an insurance contract because of his attire and office signage. By wearing an ABC Insurance jacket and having an ABC Insurance sign over his rented office, William creates a visual representation of his affiliation with the insurance company. This can lead the customer to assume that he has the authority to conduct insurance-related activities.