Synovial joints may be classified into subtypes according to :
(a) shape
(b) amount of synovial fluid
(c) number of bones
(d) amount of cartilage

Options

A. shape
B. amount of synovial fluid
C. number of bones
D. amount of cartilage

Explanation

To solve the question regarding the classification of synovial joints, we will analyze each option provided:

1. **Understanding Synovial Joints**: Synovial joints are a type of joint that allows for a wide range of motion. They are characterized by the presence of a synovial cavity filled with synovial fluid, which lubricates the joint.

2. **Analyzing the Options**:
- **(a) Shape**: Synovial joints can be classified based on their shape, such as ball-and-socket joints (like the shoulder), hinge joints (like the elbow), pivot joints, and others. This classification is based on the structural characteristics of the joint.
- **(b) Amount of synovial fluid**: While synovial fluid is important for the function of synovial joints, the amount of fluid does not classify the joints into subtypes.
- **(c) Number of bones**: The number of bones involved in a joint does not determine the subtype of synovial joints. For example, both the elbow and knee involve two bones but are classified differently based on their shape.
- **(d) Amount of cartilage**: Similar to synovial fluid, the amount of cartilage does not serve as a basis for classifying synovial joints into subtypes.

3. **Conclusion**: Based on the analysis, synovial joints are classified into subtypes primarily according to their shape. Therefore, the correct answer is option **(a) shape**.

### Final Answer:
The classification of synovial joints into subtypes is based on their **shape** (Option a).

---