The correct answer is B) chemically test the condensate for chloride content. The reasoning is that a salinity alarm indicating 2.5 grains per gallon at the main condensate pump discharge suggests potential condenser tube leakage and seawater intrusion into the condensate system. The first step should be to chemically test the condensate to confirm the chloride content, as this will help identify the source of the elevated salinity. The other options are incorrect because: A) opening the recirculating valve would not address the root cause, C) increasing hydrazine dosage would not resolve the seawater intrusion, and D) blowing down the boilers and adding makeup water would not identify the source of the salinity issue.