The correct answer is B) Thermal overload relay. A thermal overload relay is designed to protect an electric motor from damage caused by excessive current and overheating, which can occur if one phase of a three-phase motor experiences an open circuit. The thermal overload relay continuously monitors the motor current and will automatically shut down the motor if the current exceeds a preset limit, preventing the motor from being damaged due to the unbalanced load caused by the open phase. The other options are not suitable for protecting against an open phase condition. Overspeed trip (A) and magnetic blowout coil (D) are not designed to address this specific issue. A three-pole safety switch (C) would remove power from the motor, but would not automatically detect and respond to the open phase condition.