From the given following sol how many can coagulate silica acid sol?
`Fe(OH)_(3),Ca(OH)_(2),Al(OH)_(3),` starch, clay, `As_(2)S_(3),CdS,` basic dye.

Options

A. 4
B. 3
C. 2
D. 8

Explanation

To determine how many of the given substances can coagulate silica acid sol, we need to understand the nature of the silica acid sol and the charges of the substances provided.

### Step-by-Step Solution:

1. **Identify the Charge of Silica Acid Sol:**
- Silica acid sol is negatively charged. This is important because negatively charged sols can be coagulated by positively charged particles.

2. **List the Given Substances:**
- The substances provided are:
- Fe(OH)₃ (Iron(III) hydroxide)
- Ca(OH)₂ (Calcium hydroxide)
- Al(OH)₃ (Aluminum hydroxide)
- Starch
- Clay
- As₂S₃ (Arsenic sulfide)
- CdS (Cadmium sulfide)
- Basic dye

3. **Determine the Charge of Each Substance:**
- **Fe(OH)₃:** This is a positively charged sol due to Fe³⁺ ions.
- **Ca(OH)₂:** This is also a positively charged sol due to Ca²⁺ ions.
- **Al(OH)₃:** This is positively charged due to Al³⁺ ions.
- **Starch:** This is a negatively charged sol.
- **Clay:** This is also a negatively charged sol.
- **As₂S₃:** This is a negatively charged sol.
- **CdS:** This is a negatively charged sol.
- **Basic dye:** This is a positively charged sol.

4. **Count the Positively Charged Substances:**
- From the analysis, the positively charged substances are:
- Fe(OH)₃
- Ca(OH)₂
- Al(OH)₃
- Basic dye
- Therefore, there are 4 positively charged substances.

5. **Conclusion:**
- The total number of substances that can coagulate silica acid sol is **4**.

### Final Answer:
4 substances can coagulate silica acid sol.