Which bone aligns with the thumb on the lower arm?

The radius is the bone that aligns with the thumb on the lower arm. It is one of the two major bones in the forearm, the other being the ulna. The radius is positioned on the lateral side of the forearm when the palm is facing upwards, which corresponds to the thumb side in anatomical terms. This alignment allows for the thumb's articulation with the wrist and facilitates various movements, such as gripping and thumb opposition.

The ulna, while also a bone of the forearm, is located on the opposite side, aligning more closely with the little finger. The humerus, found in the upper arm, connects to the shoulder and the elbow, while the scapula is part of the shoulder girdle and does not directly relate to the bones of the forearm. Thus, in context to the question, the radius is definitively the bone that aligns with the thumb.