The correct answer is C) replace the zincs if deteriorated by 50%. The reason this is the correct answer is that zinc pencils or plates in a refrigerating system condenser are meant to serve as sacrificial anodes, protecting the condenser from corrosion. As the zinc deteriorates over time, it should be replaced once it has lost around 50% of its original mass to continue providing adequate corrosion protection. The other options are incorrect because: A) renewing the plates at each inspection is unnecessary, B) filing the plates would not be an appropriate way to address corrosion, and D) painting and insulating the plates would interfere with their intended function as sacrificial anodes.