Correct option is D
Square threads are commonly used on machine spindles because they are highly efficient for power transmission. Their geometry allows force to be transmitted parallel to the axis, minimizing friction losses compared to other thread forms. This makes them ideal for applications like screw jacks, presses, and machine spindles where smooth motion and high load transmission are required.
In contrast, knuckle threads are used in rough applications like railway couplings, buttress threads are designed for one-directional heavy loads, and unified threads are general-purpose fastening threads. These are not suitable for spindle applications where efficiency and precision are critical.
Thus, due to their low friction, high efficiency, and ability to handle axial loads effectively, square threads are preferred for machine spindles.
