The correct answer is C) the motor will continue to run, but will vibrate and have reduced torque. In a three-phase induction motor, if one of the supply lines develops an open, the motor will continue to run, but it will experience an imbalance in the three-phase system. This imbalance will result in the motor vibrating and having reduced torque output. The motor will not stop (option B) because the other two phases can still provide enough power to keep the motor running, although at a reduced capacity. The motor will not run cooler (option A) due to the imbalance, and it will not speed up (option D) since the number of poles remains the same.