In the experiment on elecro-osmosis, in which of the following the level of the dispersion medium will fall on the cathode side ?

Options

A. Gold sol
B. Starch sol
C. `Fe(OH)_(3)` sol
D. `As_(2)S_(3)`sol

Explanation

To solve the question regarding the electro-osmosis experiment and determine which dispersion medium level will fall on the cathode side, we can follow these steps:

### Step-by-Step Solution:

1. **Understand Electro-osmosis**: Electro-osmosis is a phenomenon where the movement of liquid (dispersion medium) occurs due to an applied electric field. In this process, the charged particles in the dispersion medium move towards the electrode of opposite charge.

2. **Identify Charges**: In the context of electro-osmosis:
- The **dispersion medium** is negatively charged.
- The **dispersion phase** (the particles dispersed in the medium) is positively charged.

3. **Determine the Cathode Charge**: The cathode is the electrode where reduction occurs and is negatively charged. Therefore, positively charged particles in the dispersion phase will be attracted towards the cathode.

4. **Analyze the Options**: We need to identify which of the given options contains positively charged particles that will migrate towards the cathode. The options provided in the question include:
- Gold solution (negatively charged)
- Star solution (negatively charged)
- A2S3 (negatively charged)
- FeOS3 (positively charged)

5. **Select the Correct Option**: Among the options, only **FeOS3** is positively charged. Therefore, when the electric field is applied, the level of the dispersion medium will fall on the cathode side due to the movement of FeOS3 towards the cathode.

6. **Conclusion**: The correct answer is **Option 3: FeOS3**.