Explanation:

The correct answer is the cooler.
• Double comparatives are phrases commonly used to express proportionate increase or decrease, to say that when something increases or decreases, it causes something else to change.
• We can make comparisons using adjectives, adverbs and nouns.
• Example:
• The angrier the teacher is, the worst I feel. X
• The angrier the teacher is, the worse I feel."
• The sentence talks about the effect of altitude on temperature.
• As per the points given above, 'the cooler' is the correct choice in the part given in the 1st option.