The correct answer is B) steam admitted to a cold pipe. Water hammer in a steam heating system is caused by the rapid condensation of steam when it comes into contact with a cold surface, such as a cold pipe. When steam is suddenly admitted to a cold pipe, the steam rapidly condenses, creating a pressure wave that can cause damage to the system. This is known as water hammer. The other options are incorrect because: A) and C) filling the auxiliary boiler with cold or hot water would not directly cause water hammer in the steam heating system, and D) draining a soot blower line before cracking the steam supply valve would not necessarily lead to water hammer.