In a reaction catalyst changes
a) Physically `" "` b) qualitatively
c) Chemically `" "` d) quantitatively

Options

A. a,b
B. b,c
C. d,a
D. a,c

Answer

To answer the question regarding how a catalyst changes in a reaction, we need to analyze the properties of a catalyst.

### Step-by-Step Solution:

1. **Understanding the Definition of a Catalyst**:
- A catalyst is a substance that increases the rate of a chemical reaction without undergoing any permanent chemical change itself. This means that after the reaction, the catalyst can be recovered in its original form.

2. **Physical Changes**:
- A catalyst can undergo physical changes, such as changes in its state (solid, liquid, gas) or appearance (color, size). However, these changes do not affect its chemical composition.

3. **Qualitative Changes**:
- A catalyst may also exhibit qualitative changes, meaning that its properties might change in terms of how it interacts with reactants or products, but it does not change chemically.

4. **Chemical Changes**:
- A catalyst does not undergo any chemical change during the reaction. It remains unchanged in its chemical structure and composition after the reaction.

5. **Quantitative Changes**:
- Similarly, a catalyst does not change in quantity. The amount of catalyst used in the reaction remains the same before and after the reaction.

6. **Conclusion**:
- Based on the analysis, the correct options for how a catalyst changes in a reaction are:
- a) Physically (it can change in appearance or state)
- b) Qualitatively (it can change in how it interacts with reactants)
- Therefore, the catalyst does not change chemically or quantitatively.

### Final Answer:
- The correct options are:
a) Physically
b) Qualitatively