On a cold, winter morning the most likely place for radiation fog to form is:

The answer is - in a valley.

Radiation fog is a type of fog that is getting formed after the sunset, when the land starts to cool down under the infrared thermal radiation, but also it needs some specific conditions to form, like clear sky, and calm conditions where the wind is absent from the given area. The cooling of the ground causes also cooling of the adjacent air by conduction, which in turn causes the air temperature to fall down to the dew point, which as an end result produces the radiation fog.