Final answer:
When Marc is sitting at his desk with good posture, it means he is sitting upright with his back straight and shoulders relaxed, which is a healthy and beneficial position to maintain.
Step-by-step explanation:
When someone has good posture, it means they are sitting or standing in a way that aligns their body properly. This means that their center of gravity is directly above the pivot point in their hips. Good posture helps to avoid strain and damage to the back and disks. In the case of Marc sitting at his desk with good posture, it suggests that he is sitting upright with his back straight and shoulders relaxed, which is a healthy and beneficial position to maintain.