The skin of frog is smooth and slippery due to the presence of 

Options

A. mucus in skin
B. mucus in stomach
C. muscles in skin
D. all the above

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks why the skin of a frog is smooth and slippery.

2. **Identifying the Options**: The options provided are:
- Mucous and skin
- Mucous and stomach
- Muscle and skin
- All of the above

3. **Analyzing the Options**:
- **Mucous and skin**: Mucous is a slippery, stringy fluid produced by the skin's lining tissue. This option seems plausible as mucous is known to make surfaces slippery.
- **Mucous and stomach**: While mucous can be found in the stomach, it does not contribute to the slipperiness of the skin. This option can be ruled out.
- **Muscle and skin**: Muscle does not contribute to the slipperiness of the skin. This option can also be ruled out.
- **All of the above**: Since two of the options are incorrect, this option cannot be correct.

4. **Conclusion**: The skin of the frog is smooth and slippery primarily due to the presence of mucous produced by the skin itself. Therefore, the correct answer is **Mucous and skin**.

5. **Final Answer**: The skin of the frog is smooth and slippery due to the presence of **mucous**.