Answer:
Option C
Step-by-step explanation:
A control group with in an experiment is the subject which is not exposed to any treatment just in order to compare the result of treated group with non-treated group.
Here in this experiment, showers which are receiving water containing mildew represents the control group as the mildew growth on such showers will be compared with the treated group i.e the showers which are sprayed to reduce the growth of mildew.
Hence, Option C is correct