The correct option is D much
The context refers to a comparative situation (in emergencies as opposed to when there is no emergency). So, option D is correct as the adverb 'much' indicates 'to a great extent or often'. It adds to the verb 'react' and gives us more information on the speed with which the policemen react.
Option A 'many' is incorrect as it is a quantifier/determiner which indicates the number of something.
Option B 'a lot of' is incorrect as this quantifier indicates the quantity.
Option C 'some' is incorrect as it indicates to some extent, whereas the context implies a large extent.
So, option D is correct.