What bone allows the arm to rotate due to its circular head?

The radius is the correct choice because it features a circular head that allows for rotational movement at the proximal radioulnar joint. This unique structure enables the radius to articulate with the humerus and ulna, facilitating the arm's ability to rotate and perform actions such as pronation and supination, which are essential for functions like turning the palm upwards or downwards.

The ulna, while critical for forearm stability and movement, primarily serves as a hinge joint at the elbow and does not have the same rotational capacity as the radius. The humerus, the upper arm bone, does not contribute to the rotation of the forearm; rather, it connects the shoulder to the elbow but does not feature a circular head itself in terms of forearm rotation. The clavicle acts as a strut to connect the arm to the body but does not play a direct role in the rotational movement of the arm. Therefore, the radius's circular head is pivotal in enabling the arm's rotation.