The speed regulation of a DC motor is given by the formula:

Speed Regulation = (NnL - NfL) / NfL

where NnL is the no-load speed of the motor and NfL is the full-load speed of the motor.

The speed regulation measures the change in speed of the motor from no-load to full-load conditions. It provides an indication of the motorâ€™s ability to maintain a relatively constant speed under varying load conditions.

In the formula, (NnL - NfL) represents the change in speed between no-load and full-load, and dividing it by NfL provides a percentage representation of this change relative to the full-load speed.

Therefore, the correct answer is C: (NnL - NfL)/NfL.