Final answer:
A muscle with its origin at the humerus and insertion at the ulna would act on the elbow joint (option C). The elbow joint allows for movements of flexing or extending the forearm due to the articulation between the humerus and forearm bones like the ulna.
Step-by-step explanation:
If a muscle's origin is at the humerus and its insertion is at the ulna, the joint it would flex or extend is the Elbow joint. This is because the humerus is the bone of the upper arm, and the ulna is one of the bones in the forearm. The humerus and ulna are connected at the elbow joint which allows for flexion (bending) and extension (straightening) of the forearm.
For example, the biceps brachii is a muscle with its origin on the scapula (not the humerus), and it inserts on the radial tuberosity of the radius (close to the ulna) - when it contracts, it flexes the elbow joint. Conversely, the triceps brachii muscle has its origin on the humerus and inserts on the olecranon of the ulna. When the triceps brachii contracts, it extends the elbow joint. Therefore, the correct answer to the student's question is C: Elbow joint.
Key Points:
- The humerus is the upper arm bone that articulates with the forearm bones at the elbow.
- The ulna is one of the forearm bones that forms part of the elbow joint.
- Muscles spanning between the humerus and ulna would act on the elbow joint, not the hip, shoulder, or knee joints.
Hence, the answer is option C.