**Core Concept**
The question is testing the classification of joints based on their mobility. In anatomy, joints are categorized into three main types: synarthroses (immovable), amphiarthroses (slightly movable), and diarthroses (freely movable). The correct answer will be a type of joint that belongs to the synarthrosis category.

**Why the Correct Answer is Right**
An immovable joint, also known as a synarthrosis, is a type of joint where the bones are completely fused together, with no movement or space between them. This type of joint is typically found in areas where stability and protection are essential, such as the sutures of the skull. The bones in a synarthrosis joint are held together by fibrous connective tissue, which provides strength and rigidity.

**Why Each Wrong Option is Incorrect**
**Option A:** A hinge joint is an example of a diarthrosis, which allows for movement in one plane. This is the opposite of an immovable joint.
**Option B:** A pivot joint is also a type of diarthrosis, allowing for rotational movement around a single axis. This is not an immovable joint.
**Option C:** A ball-and-socket joint is another type of diarthrosis, allowing for movement in multiple planes. This is not an immovable joint.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that synarthroses, or immovable joints, are characterized by complete fusion of the bones, with no space or movement between them. This is in contrast to amphiarthroses, which have some movement, and diarthroses, which allow for free movement.

**Correct Answer:** C. Sutures. Sutures are immovable joints that connect the bones of the skull.