The answer is: C. The clear day, which is veiled in shadow
In the book “To Build a Fire” by Jack London, the man is warned of not traveling alone in Yukon when the temperature can get so low, he later understands the advice of the old-timer, that man does not have the instinct to survive the danger there, for this reason the clear day, were the sun shines, which is veiled in shadow, is the most proper setting to describe the theme.