The incorrect statement among the following is :

Options

A. `VOSO_4` is a reducing agent
B. `Cr_2 O_3` is an amphoteric oxide
C. `RuO_4` is an oxidizing agent
D. Red colour of ruby is due to the presence of `Co^(3+)`

Answer

To solve the question of identifying the incorrect statement among the given options, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Analyzing Statement 1: "Rudinium tetraoxide is an oxidizing agent."**
- **Explanation:** An oxidizing agent is a substance that causes oxidation by accepting electrons from another substance. Rudinium tetraoxide (RuO₄) is known to be a strong oxidizing agent. Therefore, this statement is **correct**.

2. **Analyzing Statement 2: "Osmium tetraoxide is an oxidizing agent."**
- **Explanation:** Osmium tetraoxide (OsO₄) is also a well-known oxidizing agent. It is used to convert alkenes into diols through oxidation. Thus, this statement is **correct**.

3. **Analyzing Statement 3: "Chromium oxide is amphoteric."**
- **Explanation:** Chromium oxide (CrO₃) can react with both acids and bases, which defines it as amphoteric. The oxidation state of chromium in chromium oxide is +3, and it can exhibit various oxidation states. Therefore, this statement is **correct**.

4. **Analyzing Statement 4: "The red color of ruby is due to cobalt 3 plus."**
- **Explanation:** Rubies are primarily composed of aluminum oxide (Al₂O₃) with trace amounts of chromium (Cr³⁺). The red color of rubies is attributed to the presence of chromium ions, not cobalt ions. Therefore, this statement is **incorrect**.

### Conclusion:
The incorrect statement among the options is **Statement 4: "The red color of ruby is due to cobalt 3 plus."**