D.) Instinct is an important part of survival
The dog represents pure instinct, a trait necessary for survival in the extremely cold Yukon at this time of year. ("But the dog knew; all its ancestry knew, and it had inherited the knowledge. ")
The man never uses instinct, which would inform him without thinking that certain actions are dangerous while others are absolutely necessary for survival. The reason the man never uses instinct is explained in this sentence: "Possibly all the generations of his ancestry had been ignorant of cold, of real cold, of cold one hundred and seven degrees below freezing-point."