The correct answer is B) An increased pressure drop across the strainer (more differential). When a heavy fuel oil purifier supply pump suction strainer becomes clogged, it will create a higher pressure drop or differential across the strainer. This increased pressure drop indicates that the strainer needs to be cleaned, as the clogged strainer is restricting the flow of fuel oil to the purifier. The other options are incorrect because an increase in fuel oil temperature, a decrease in pressure drop, or a drop in purifier bowl speed would not be the primary indication that the strainer needs cleaning. The increased pressure drop across the strainer is the key indicator that maintenance is required.