The correct answer is: A. Humerus and pectoral girdle
A hinge joint is a type of synovial joint that allows for movement in one plane only, like a door swinging on its hinges. The humerus and pectoral girdle are connected by the shoulder joint, which is a ball-and-socket joint that allows for a wide range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction. However, the primary movement of the shoulder joint is flexion and extension, which is why it is considered a hinge joint.
The other options are incorrect because they do not involve hinge joints. The femur and acetabulum are connected by the hip joint, which is a ball-and-socket joint that allows for a wide range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction. The humerus and radio-ulna are connected by the elbow joint, which is a hinge joint that allows for flexion and extension. The femur and pelvic girdle are connected by the hip joint, which is a ball-and-socket joint that allows for a wide range of motion, including flexion, extension, abduction, adduction, rotation, and circumduction.