Opposition is the movement of any bone in the body that opposes another bone in the body.

Choice A reason: Parathyroid hormone (PTH) is secreted by the parathyroid glands in response to low blood calcium levels. It stimulates osteoclast activity, leading to bone resorption and the release of calcium into the bloodstream. PTH also enhances calcium reabsorption in the kidneys and activates vitamin D to increase intestinal calcium absorption.

Choice B reason: Thyroxine, produced by the thyroid gland, primarily regulates metabolism. While it influences bone growth and development indirectly, it does not directly stimulate osteoclast activity or calcium release.

Choice C reason: Osteocalcin is a non-collagenous protein secreted by osteoblasts and is involved in bone mineralization and calcium ion homeostasis. However, it does not stimulate osteoclast activity or increase blood calcium levels.

Choice D reason: Calcitonin is secreted by the parafollicular cells of the thyroid gland in response to high blood calcium levels. It inhibits osteoclast activity, thereby reducing bone resorption and lowering blood calcium levels. It acts in opposition to PTH.

Choice A reason: Circumduction is a circular movement that combines flexion, extension, abduction, and adduction. It is typically seen in ball-and-socket joints like the shoulder or hip, not in the neck. It does not describe the backward bending of the head.

Choice B reason: Rotation refers to the movement of a bone around its longitudinal axis, such as turning the head side to side. It does not involve bending the head backward.

Choice C reason: Hyperextension occurs when a joint is extended beyond its normal anatomical position. In the case of the head, bending it backward past the neutral anatomical position is a classic example of hyperextension.

Choice D reason: Flexion is the movement that decreases the angle between two body parts, such as bending the head forward toward the chest. It is the opposite of the movement described in the question.