The correct answer is B) When the smoke indicator (periscope) shows clear. The determining factor in deciding how many revolutions of rotation of a soot blower element are required is when the smoke indicator (periscope) shows clear. This is because the purpose of the soot blower is to remove soot and ash buildup from the boiler tubes, and the smoke indicator provides a visual indication of when the tubes have been sufficiently cleared. Rotating the soot blower element until the smoke indicator shows clear ensures that the tubes have been properly cleaned. The other answer choices are incorrect because they do not directly relate to the primary purpose of the soot blower, which is to remove soot and ash from the boiler tubes. The scavenging air indicator and the steam supply manifold drains are not directly relevant to determining the number of soot blower revolutions required.