Which simple machine is primarily used to lift heavy objects by converting rotational force into linear force?

The correct choice is a screw, as this simple machine functions effectively by converting rotational motion into linear motion, which is essential for lifting heavy objects. When you turn a screw, the rotational force applied causes the screw to move linearly along its axis as it penetrates or lifts against the material, such as wood or soil. This action allows for efficient lifting and securing of heavy loads with the help of relatively less effort.

In the context of other simple machines mentioned: while a lever can also be used to lift heavy objects, it operates by distributing weight, rather than converting rotation to linear movement. The wheel and axle is primarily designed to reduce friction and facilitate movement rather than specifically lifting. An inclined plane helps in lifting objects by reducing the required force, but it does not convert rotational force into linear force like a screw does.