The correct answer is A) Purge the furnace of combustible gases. This is the correct answer because it is a critical safety precaution to take before lighting off a boiler. Boilers contain combustible gases that can build up during shutdown periods. Purging the furnace ensures these gases are cleared out, preventing the risk of an explosion or uncontrolled ignition when the boiler is fired up. The other options, while potentially important maintenance steps, do not directly address the safety hazard of combustible gases in the furnace prior to ignition. Securing drains or closing air registers does not eliminate this fundamental risk in the same way that purging the furnace does.