Final answer:
The correct action that defines extending the metacarpophalangeal joint of the thumb is moving the thumb away from the palm, aligning it with the plane of the palm, which corresponds to Option A.
Step-by-step explanation:
When it comes to the motions of the thumb, particularly the metacarpophalangeal (MP) joints, extension refers to the movement in which the thumb is moved away from the palm, aligning with the plane of the palm. This is different from flexion, which is when the thumb is brought towards the index finger or into the palm. The options given describe actions happening at these joints. Notably, the thumb does not have a proximal interphalangeal joint; the thumb has only one interphalangeal joint, unlike the fingers which have both proximal and distal interphalangeal joints. Therefore, the correct answer that defines extending the metacarpophalangeal joint of the thumb is Option A) Extending the metacarpophalangeal joint of the thumb.