The correct answer is: D. stainless steel
Stainless steel is the most common material used for the shaft of a synchronous motor. It is a strong, durable material that can withstand the high temperatures and pressures generated by the motor. Stainless steel is also resistant to corrosion, which is important in a motor that is exposed to moisture and other contaminants.
Mild steel is a type of steel that is not as strong as stainless steel. It is also more susceptible to corrosion. Chrome steel is a type of steel that is coated with chromium. This coating makes the steel more resistant to corrosion, but it does not make it as strong as stainless steel. Alnico is a type of alloy that is made of aluminum, nickel, and cobalt. It is a strong, magnetic material, but it is not as corrosion-resistant as stainless steel.
In conclusion, the shaft of a synchronous motor is most commonly made of stainless steel. This is because stainless steel is a strong, durable material that can withstand the high temperatures and pressures generated by the motor. Stainless steel is also resistant to corrosion, which is important in a motor that is exposed to moisture and other contaminants.