Final answer:
Jack London describes the Yukon in paragraph 20 of 'To Build a Fire' using the image of the coldness of space affecting the unprotected tip of the planet, highlighting the harsh and extreme environment.
Step-by-step explanation:
The image that Jack London uses to describe the Yukon in paragraph 20 of "To Build a Fire" is A. "The cold of space smote the unprotected tip of the planet..." This vivid description is used to convey the extreme cold and harsh environment of the Yukon, which is so intense that it feels as if the cold from space itself is directly hitting the planet's surface where the man is located. It emphasizes the idea that humans are vulnerable and almost alien to such an environment.