The correct answer is B) An increased pressure drop across the strainer on service (more differential). The primary indication that a fuel oil service system booster pump duplex suction strainer needs to be shifted over and cleaned is an increased pressure drop across the strainer element. As the strainer becomes clogged with debris, the resistance to flow increases, resulting in a higher pressure drop across the strainer. This increased differential pressure signals that the strainer needs to be switched over and cleaned to maintain proper fuel oil system operation. The other options are incorrect because a decreased pressure drop (A) would indicate the strainer is clean, a drop in pump speed (C) could have other causes, and an increase in pump discharge pressure (D) may not necessarily indicate a strainer issue.