Select the correct statement

Options

A. a) Viscosity in gases arises primarily from the molecular diffusion that transport momentum layer with respect to the other
B. b) Viscosity of ideal gas increases at temperature increases
C. c) Viscosity of liquids decreases with the rise in temperature
D. d) At critical temperature the surface of separation between liquid and gas disappear and their viscosity is same

Explanation

To solve the question, we need to evaluate each of the four statements given regarding viscosity and determine which ones are correct.

### Step-by-Step Solution:

1. **Evaluate Statement A**:
- **Statement**: "Viscosity of gases arises primarily from the molecular diffusion that transport momentum layer with respect to the other."
- **Analysis**: Viscosity in gases is indeed related to molecular diffusion, as it involves the transfer of momentum between layers of gas molecules. This statement is correct.

2. **Evaluate Statement B**:
- **Statement**: "Viscosity of ideal gas increases at a temperature increases."
- **Analysis**: For an ideal gas, the viscosity is proportional to the square root of the temperature (T^(3/2)). Therefore, as temperature increases, the viscosity also increases. This statement is correct.

3. **Evaluate Statement C**:
- **Statement**: "Viscosity of liquids decreases with the rise in temperature."
- **Analysis**: For liquids, as temperature increases, the viscosity typically decreases due to increased molecular motion. This statement is correct.

4. **Evaluate Statement D**:
- **Statement**: "At a critical temperature, the surface of separation between liquid and gas disappear and their viscosity is the same."
- **Analysis**: At the critical temperature, the densities of the liquid and gas become equal, but their viscosities are not the same. Liquids generally have higher viscosity than gases at the same temperature. Therefore, this statement is incorrect.

### Conclusion:
From the analysis, we find that:
- Statement A is correct.
- Statement B is correct.
- Statement C is correct.
- Statement D is incorrect.

**Final Answer**: The correct statements are A, B, and C.