The correct answer is A) 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 is due to the polarization of the insulation material, which takes time to fully charge, resulting in a slowly increasing resistance reading on the megohmmeter. The other options are incorrect because: B) good conductor resistance would not cause a gradual rise, C) the inductive reactance of the windings is not relevant to insulation testing, and D) the leakage of current along the surface of dirty insulation would not result in a gradual rise, but rather a steady or decreasing reading.