You get lost in extremely cold weather. You are unable to start a fire. Which of the following is the best way to survive?

Wrapping up in a thermal foil blanket is an effective method of survival in extremely cold weather, especially when you are unable to start a fire. Thermal foil blankets, also known as space blankets, are designed to retain body heat. They work by reflecting up to 90% of your body heat back, which is crucial in preventing hypothermia in cold conditions. Keeping your body temperature stable is the most critical factor in survival when exposed to the elements.

While building a snow shelter can provide some protection from the wind and possibly trap heat, it requires a significant amount of energy and time, which can be a disadvantage if you're already cold and exhausted. Signaling for help with a whistle is useful for gaining attention, but it doesn't address the immediate need for warmth and shelter. Walking to find help might seem like a proactive approach, but it can be hazardous if the terrain is treacherous or if you are already disoriented, potentially leading to more exposure to the cold.

In summary, the best immediate action in this situation is to use a thermal foil blanket to conserve body heat and protect against the chilling effects of extreme cold.