The correct answer is B) priming and carryover. When a boiler with a higher steam pressure is brought on the line with another boiler already operating, the sudden pressure differential can cause water to be carried over from the higher pressure boiler into the lower pressure boiler. This is known as "priming and carryover," which can damage the internal components of the lower pressure boiler due to the uncontrolled introduction of water. The other options are incorrect because: A) Low water is not a direct consequence of the pressure differential, but could be a separate issue. C) An overloaded superheater is not a concern in this scenario, as the issue is related to the boiler pressure, not the superheater. D) Thermal shock is not the primary risk in this situation, as the issue is the potential for water carryover, not a sudden temperature change.