Answer:
The narrator had a difficult time thinking because of the cold.
Step-by-step explanation:
When we say that someone is doing something at a snail's pace, we are actually saying that they are doing something very slowly. Usually, we want that person to be faster and more efficient at what they are doing.
In the given passage, the narrator says that the cold is slowing his thoughts to a snail's pace. What he is saying is that he has a difficult time thinking because of the cold. When we're cold, all we can think about is how we want to feel warm again. Every other thought is less important, which makes cold very distracting.
This is why the second option is the correct one.