The correct answer is B) Sudden large increase in the firing rate. The reason this is the correct answer is that a sudden large increase in the firing rate of the boiler can cause a rapid increase in steam production in the economizer. This can lead to steam formation in the economizer, as the economizer may not be able to effectively transfer the excess heat to the incoming feedwater. The other options are incorrect because: A) Soot buildup on the gill rings would restrict heat transfer and reduce steam production, not cause steam formation. C) Excessive water flow rates would actually reduce the risk of steam formation in the economizer. D) An open main feed pump recirculating line would not directly cause steam formation in the economizer.