The correct option is B cooler
The given sentence requires the most suitable form of an adjective. The word 'much' is a comparative degree of adverb and is often accompanied by a comparative degree of an adjective. Hence, we can say - 'After a week of extreme heat, it is much cooler now.' Thus the correct answer is C- Cooler. Option A is incorrect, as 'much cool' is grammatically incorrect for the reason explained above. Option B is incorrect, as 'coolest' is the superlative degree of adjective and can't be paired with 'much'. Option D is incorrect, as option C is the answer.