The correct answer is B) the dielectric-absorption effect of the insulation. When a megohmmeter is used to test insulation, the gradual rise of the pointer reading as a result of continued cranking is caused by the dielectric-absorption effect of the insulation. This effect occurs because the insulation material has a property called dielectric absorption, which causes it to slowly absorb and store electrical energy over time when subjected to an applied voltage. As the megohmmeter continues to crank, the insulation gradually charges up, resulting in the gradual rise of the pointer reading. The other options are incorrect because: A) leakage of current along the surface of dirty insulation would result in a decreasing pointer reading, not a gradual rise; C) good conductor resistance would not cause a gradual rise in the pointer reading; and D) the inductive reactance of the windings is not relevant to the insulation testing using a megohmmeter.