The activity and selectivity of zeolites as catalyst is based on :-

Options

A. Their pore size
B. Size of their cavities on the surface
C. Both
D. None

Answer

### Step-by-Step Solution:

1. **Understanding Zeolites**:
- Zeolites are crystalline aluminosilicates that have a porous structure. They are widely used as catalysts in various chemical processes, particularly in the petrochemical industry.

2. **Shape Selectivity**:
- The catalytic activity and selectivity of zeolites are primarily due to their shape-selective properties. This means that the structure of the zeolite allows only certain molecules to enter their pores and react, while excluding others based on size and shape.

3. **Pore Size**:
- The pore sizes of zeolites typically range from 260 picometers to 740 picometers. This specific range allows zeolites to selectively catalyze reactions involving molecules that fit within their pores, enhancing the efficiency of the catalytic process.

4. **Cavities and Surface Properties**:
- The cavities formed by the arrangement of the zeolite structure also play a crucial role in determining which reactions can occur. The size and shape of these cavities influence the types of reactants that can access the active sites within the zeolite.

5. **Conclusion**:
- Therefore, the activity and selectivity of zeolites as catalysts are based on their pore size and the shape of their cavities. This unique characteristic allows zeolites to be effective catalysts in specific reactions, particularly in the petrochemical industry.

### Final Answer:
The activity and selectivity of zeolites as catalysts are based on their pore size and the shape of their cavities.

---