The correct answer is D) overload relay thermal heater. The "OL" symbol in the power circuit on lines T1 and T3 to the motor represents the overload relay thermal heater. The overload relay is a critical safety device that monitors the current flowing through the motor and will interrupt the power supply if an overload condition is detected, protecting the motor from damage. The thermal heater element is the sensing component that triggers the overload relay to open the circuit when excessive current is drawn by the motor. The other answer choices are incorrect because: A) the overload relay contacts are normally open, not normally closed; B) a fusible link is a different type of overcurrent protection device; and C) the overload relay magnetic coil is a different component that actuates the relay mechanism, not the sensing element.