A fog is most likely to develop when a

Fog forms when air is cooled to its dew point, causing water vapour to condense into tiny suspended water droplets near the ground. The type of fog described here is advection fog, which occurs when a mass of warm, moist air moves horizontally over a colder surface.

When a warm moist wind blows over a cold ocean current, the lower layers of the air mass are rapidly chilled by contact with the cold water. As the air temperature drops to the dew point, the moisture in the warm air condenses into fog. This is common along coastlines where warm and cold currents meet, such as off the coasts of Namibia (Benguela Current) and California (California Current).

The other scenarios are less effective at producing fog:

• A cold, dry wind over a warm current may pick up some moisture but is unlikely to reach saturation quickly enough to form dense fog.
• A warm moist wind over a warm current experiences little cooling, so condensation does not occur.
• A warm, dry wind over a cold current lacks sufficient moisture to produce fog even if cooling occurs.

The correct answer is that fog develops when a warm moist wind blows over a cold current.