Final answer:
The MP joints of the thumb allow it to perform flexion, bringing the thumb into the palm (Option A). There is no proximal interphalangeal joint in the thumb, making Option C and D inapplicable.
Step-by-step explanation:
The MP (metacarpophalangeal) joints of the thumb are the biaxial joints that allow the thumb to move in various directions including flexion and extension. Flexion of the MP joint is a movement that brings the thumb towards the palm or across the palm to meet the base or pad of the little finger, essentially making a fist. Extension is the opposite movement, where the thumb is moved away from the hand's palm, straightening out the thumb.
When considering the options provided in the question, flexing the metacarpophalangeal joint (the knuckle) of the thumb is described as Option A. Importantly, the thumb does not possess a proximal interphalangeal joint like the other fingers; it has only one phalangeal joint. Therefore, Option C and Option D are not applicable anatomical movements for the thumb. Option B describes the opposite of flexion, which is extension.
A: Flexing the metacarpophalangeal joint of the thumb. This is because flexion is the movement that brings the thumb towards the palm or towards the base of the little finger.