The city has proposed a number of water treatment and conservation projects the cost of which raises water bills high enough so that even environmentalists are beginning to raise alarms. a. the cost of which raises water bills high enough so that.
b. at a cost raising water bills so high that.
c. at a cost which raises water bills high enough so.
d. whose cost will raise water bills so high that.
e. whose cost will raise water bills high enough so that.

b. at a cost raising water bills so high that.

Explanation:

From the description of the sentence, it shows that the action when performed would yield the observed outcome. The cost of the project which would result to an exorbitant rise of water bills has to be questioned.

Thus the most suitable option is b, so that the correct expression would be;

The city has proposed a number of water treatment and conservation projects at a cost raising water bills so high that even environmentalists are beginning to raise alarms.

answer assertive;