The correct answer is B) Water hammer. Water hammer is a condition that can cause a boiler economizer to leak. When water suddenly stops flowing or changes direction in a piping system, it can create a rapid increase in pressure, known as water hammer. This sudden pressure spike can damage the economizer, leading to leaks. The other options are not the primary causes of economizer leaks. Low feed water pressure (A) and high feed water temperatures (C) may affect the overall boiler operation, but they do not directly cause leaks in the economizer. High stack gas temperatures (D) could potentially lead to thermal stress, but water hammer is the more common and direct cause of economizer leaks.