The correct answer is A) air chamber. An air chamber on the discharge side of a reciprocating feed pump helps to prevent pulsations from developing in the boiler feed water lines. The air chamber acts as a shock absorber, dampening the pulsations caused by the intermittent action of the reciprocating pump. This ensures a smoother, more consistent flow of feed water to the boiler, which is essential for proper boiler operation. The other options are incorrect because: B) a relief valve is used to prevent overpressure, not to dampen pulsations; C) a feed water regulator controls the flow rate, but does not prevent pulsations; and D) a reed valve is a type of check valve, which does not specifically address the issue of pulsations.