The correct answer is D) prevent back up of combustion gases into soot blower heads. Scavenging air is supplied to steam soot blower elements to prevent the back up of combustion gases into the soot blower heads. This is important to ensure the safe operation of the soot blowers and prevent the release of potentially hazardous gases into the boiler room. The scavenging air creates a positive pressure that keeps the combustion gases from flowing back into the soot blower elements. The other answer choices are incorrect because they do not directly address the purpose of the scavenging air, which is to prevent the backflow of combustion gases.