Introduction:
Screws are widely used in power transmission systems to convert rotary motion into linear motion or vice versa. They are commonly used in various applications such as lead screws, ball screws, worm gears, and screw jacks. The efficiency of screws used in power transmission is an important factor to consider in order to optimize the performance of the system.

High Efficiency:
The correct answer to the given question is option 'A' - high efficiency. Here's why:

1. Efficiency: Efficiency is a measure of how effectively a machine or system converts input energy to useful output energy. In the context of power transmission, efficiency represents the ratio of output power to input power. A high efficiency screw ensures that the maximum amount of input power is converted into useful output power, minimizing energy losses.

2. Power Transmission: Screws used for power transmission are designed to transmit high torque and axial forces. They are commonly used in applications where precise linear motion or force transmission is required, such as in machine tools, automotive systems, and robotics. In these applications, it is crucial to have high efficiency to ensure that the power is effectively transmitted without significant losses.

3. Friction and Mechanical Advantage: Screws rely on the principle of friction to transmit power. The efficiency of a screw is influenced by various factors such as the thread profile, lead, pitch, lubrication, and the mechanical advantage provided by the screw. By optimizing these factors, screws can be designed to have high efficiency, reducing energy losses due to friction.

4. Thread Design: The thread design of the screw plays a significant role in determining its efficiency. Screws with a high lead or pitch can provide a greater mechanical advantage, resulting in higher efficiency. Additionally, using thread profiles with low friction coefficients, such as trapezoidal or ball screw threads, can further improve the efficiency of power transmission.

Conclusion:
In conclusion, screws used for power transmission should have high efficiency. This is important to ensure that the maximum amount of input power is converted into useful output power, minimizing energy losses. Factors such as thread design, mechanical advantage, and friction play a crucial role in determining the efficiency of screws. By optimizing these factors, screws can be designed to have high efficiency, making them suitable for various power transmission applications.