In which of the following scenarios must a driver be cautious?

Being cautious while approaching a crosswalk is crucial because this is a common area where pedestrians may be present, typically crossing the street. Drivers need to be vigilant in scanning for pedestrians who may be preparing to enter the crosswalk or who are already crossing. This situation requires a heightened awareness and readiness to stop, as yielding to pedestrians is a legal requirement in most jurisdictions.

While the other scenarios also involve considerations for safety, the presence of pedestrians makes the need for caution at crosswalks particularly critical. In intersections, drivers might be focused on other vehicles; in heavy fog, visibility issues apply broadly; and while driving on a one-way street, the primary concern would be navigation and avoiding confusion, rather than immediate hazards like pedestrians. Therefore, the approach to a crosswalk highlights a specific interaction between drivers and pedestrians that warrants extra caution.