The correct answer is B) High level main condenser. The main condenser is a critical component in the steam plant, responsible for condensing the steam after it has passed through the turbine. A high level in the main condenser can indicate issues with the condensate system, potentially leading to water carryover and damage to downstream equipment. This condition requires immediate attention and priority over the other options listed, which do not pose the same level of risk to the safe operation of the steam plant. The other options, while important to monitor, are not as critical as a high level in the main condenser. Vapor issuing from the deaerating heater vent (A) may indicate air leaks or issues with the deaeration process, but does not directly threaten the steam plant's operation. Low sewage tank chlorination section level (C) and high lube oil storage tank level (D) are important for environmental and operational concerns, respectively, but do not have the same immediate impact on the steam plant's functionality as a high main condenser level.