Final answer:
The most mobile joint capable of moving in all three directions is the shoulder joint, specifically the glenohumeral joint, making it the correct answer to the question which is the shoulder joint (B).
Step-by-step explanation:
The joint in the human body that is the most mobile, able to move in all three directions, is the shoulder joint. This joint is classified as a ball-and-socket joint, where the rounded head of one bone fits into a cuplike socket of another, allowing a wide range of motion. Specifically, the glenohumeral (shoulder) joint, where the head of the humerus articulates with the glenoid cavity of the scapula, is known to allow the greatest range of motion compared to other joints like the hip joint, which is also a ball-and-socket joint but is more stable and thus less mobile.
It is also important to note that while the shoulder joint is highly mobile, it is less stable compared to more restricted joints, such as those in the carpal bones of the wrist.