In which of the situations listed will a megohmmeter give the most accurate readings?

Correct Answer: Option D — When the machine has been shut down and grounded for a period of 15 minutes.

Accurate insulation resistance testing requires complete discharge of surface and capacitive residual charges. Shutting down and grounding the electrical machine for 15 minutes ensures thermal equilibrium, dissipates trapped static electricity, and prevents false low readings per IEEE 45 maintenance standards.