Final answer:
A magnetic starter is held closed electromagnetically by the magnetic field created from the electrical current passing through its coil. Once energized, the magnetic field maintains the contactor in the closed position for the motor to operate.
Step-by-step explanation:
The magnetic starter is an electromagnetic device that is used to start and stop motors. It consists of a coil that creates a magnetic field when electricity passes through it, which in turn pulls in a contactor to close the circuit and start the motor. Once the motor is started, the magnetic starter is held closed electromagnetically.
The correct answer to the question 'The magnetic starter is held closed by what means?' is D. Electrical magnetically. This means that once the starter is energized and pulls the contactor in, it is held in place by the magnetic field produced by the coil, and not by mechanical latching, under or overvoltage conditions.
Solution a mentions a magnetic field within the interior of the solenoid, but this refers to the general properties of a magnetic field in a solenoid, using the formula B = μonI, and does not directly answer the question about how a magnetic starter is held closed.