Which of the following is NOT a main component of an automatic transmission?

The manual gear shifter is not a main component of an automatic transmission because automatic transmissions operate using a system of hydraulic controls and electronic management rather than requiring direct input from the driver in the form of a traditional manual gear shifter. In contrast, a torque converter is essential for transferring engine power to the transmission fluid, providing the necessary torque multiplication to propel the vehicle. Planetary gear sets are crucial for allowing the automatic transmission to change gear ratios seamlessly and efficiently without driver intervention. The electronic control unit (ECU) manages those functions and optimizes gear shifts based on driving conditions. Thus, the presence of a manual gear shifter is characteristic of manual transmissions, distinguishing it from the fully automatic systems governed by the other components listed.