The correct answer is B) Triple-element. The triple-element feed water regulator provides the most effective regulation of boiler water level under all operating conditions. This is because it uses three different inputs to control the feed water flow: water level, steam flow, and feed water flow. By taking into account all three of these factors, the triple-element regulator can more accurately maintain the desired water level in the boiler, even as steam demand and other conditions change. The other options, such as single-element and double-element regulators, do not have the same level of comprehensive control over the various factors affecting boiler water level.