Final answer:
When the back of the right hand is against the right thigh, the humerus is in medial rotation. This position is not described by any of the listed anatomical positions and involves inward rotation of the arm. The main muscle compartments involved in wrist movement are not primarily engaged here.
Step-by-step explanation:
If a person places the back of the right hand against the right thigh, the humerus will be in medial rotation. This positioning means that the arm is rotated inward, with the palm facing towards the thigh. In this scenario, none of the provided anatomical positions (prone with upper and lower limbs touching at sides, standing facing the observer with arms extended outward, or supine with upper and lower limbs touching at sides) accurately describe the position since the question specifically refers to placement of the hand against the thigh.
In terms of muscle activity, the superficial posterior compartment of the forearm is responsible for extending the wrist and tilting the hand away from the body, while the superficial anterior compartment of the forearm bends the wrist towards the body. These muscle groups would not be primarily engaged during the described simple medial rotation of the humerus.