The mayor boasts that the average ambulance turnaround time, the time

Premises:
Mayor - Avg ambulance turnaround time for top priority emergencies has been reduced.
The definition of top priority has been changed to remove gun shots and electrocutions - the time consuming cases.

Conclusion: Mayor's claim is a misrepresentation.

We want to strengthen the conclusion.

(A) The number of heart attacks and strokes declined this year.
Total number of incidents is immaterial.

(B) The mayor redefined the city’s financial priorities this year.
Financial priorities is out of scope.

(C) Experts disagree with the mayor’s definition of “top-priority emergency.”
The opinion of some has nothing to do with the conclusion. Note that generally only facts make conclusions stronger or weaker. Opinion of some does not.

(D) Other cities include gunshot wound cases in their category o top-priority emergencies.
There is no right or wrong way to define top-priority emergencies. We know that it included gunshot wounds last year but the definition has now been changed to not include gun shot wounds. IT doesn't make the definition wrong, just different. This difference has led to the misrepresentation.
How other cities define top-priority emergencies is irrelevant. We just care about last year vs this year in our city.

(E) One half of all of last year’s top-priority emergencies were gunshot wounds and electrocution cases.
If a big part of last year's emergencies were gunshot wounds & elec and this year, they are not included, this could explain the reduction in the average turnaround time. If last year gunshot wounds and elec accounted for a very small part of top priority emergencies, not including them would not have made much difference at all. But since a big part of last year emergencies weer gunshot wounds & elec, it does make sense that not including them will reduce the turnaround time without any actual difference on ground.

Hence (E) is correct.