Explanation:

- The passage discusses the preservation of mammoth bodies in a particular environment.

- The key focus of this sentence is on how the bodies were protected from decay.

- Option 1: "in" is typically used to indicate being enclosed or surrounded by something.

- Option 1: "in" the permafrost is the correct choice. It implies the mammoths were encased within the permafrost.

- Option 2: "at" is generally used to indicate a specific location or point, which doesn’t suit the context here.

- Option 3: "on" suggests surface connection, inappropriate since the mammoths were enclosed by permafrost.

- Option 4: "from" suggests origin or separation, not fitting for the context of being surrounded.

Based on the explanation above, .