Synovial joints are classified into six main categories based on:

a) the amount of synovial fluid found in the joint cavity.
b) their location.
c) their size.
d) the movements they allow.

The movements they allow

Explanation:

Movement joints also called synovial joints are found at various points of the appendages. These joints are characterized by bones covered with cartilage at the ends and bones being held together by tough ligaments. The synovial fluid lubricates the joint and is a shock absorber. Tendons connect muscle to the bones. Common synovial joints are hinge points, ball and socket joints and pivot joints.

The hinge joints include elbow joint and knee joint. The ball and socket include hip joint and shoulder joint. The pivot joints include joint between atlas and axis in the neural canal.

the correct option is option d