The correct answer is option 'B' - Movable joint. Movable joints are also known as synovial joints, and they allow bones to move smoothly because of a fluid that acts as a lubricant. Let's explore this in more detail.

**Movable Joints:**

Movable joints are the most common type of joint in the human body. They are found in various parts of the body, including the limbs, neck, and spine. These joints allow for a wide range of movements, such as flexion, extension, rotation, and abduction.

**Synovial Fluid and Lubrication:**

Within movable joints, there is a synovial cavity that contains synovial fluid. This fluid is a thick, viscous liquid that acts as a lubricant for the joint. It reduces friction between the bones and allows them to move smoothly against each other.

**Functions of Synovial Fluid:**

1. **Lubrication:** The synovial fluid reduces friction between the moving bones, preventing them from rubbing against each other and wearing down.

2. **Nutrient Supply:** The fluid also provides nutrients to the cartilage that covers the ends of the bones within the joint. This helps to maintain the health and integrity of the cartilage.

3. **Shock Absorption:** Synovial fluid acts as a shock absorber, cushioning the joint against impact and reducing the risk of injury.

**Structure of Movable Joints:**

Movable joints have a complex structure that allows for their range of motion and the presence of synovial fluid. The joint is surrounded by a joint capsule, which is a tough, fibrous structure that holds the bones together. Inside the joint capsule, there is a synovial membrane that secretes the synovial fluid.

**Examples of Movable Joints:**

Some examples of movable joints include:

1. **Hinge Joint:** Found in the elbow and knee, hinge joints allow movement in only one plane, like a hinge on a door.

2. **Ball and Socket Joint:** Found in the hip and shoulder, ball and socket joints allow movement in multiple directions, including rotation.

3. **Pivot Joint:** Found in the neck, pivot joints allow rotational movements.

In conclusion, movable joints allow bones to move smoothly because of the presence of synovial fluid. This fluid acts as a lubricant, reducing friction between the bones and allowing for a wide range of movements.