Oleum is

Options

A. castor Oil
B. oil of vitriol
C. fuming `H_(2)SO_(4)`
D. none of them

Explanation

**Step-by-Step Solution:**

1. **Understanding Oleum**: Oleum is a solution of sulfur trioxide (SO3) in sulfuric acid (H2SO4). It is often referred to as "fuming sulfuric acid" because it releases fumes of sulfur trioxide when exposed to air.

2. **Identifying the Components**: The main components of oleum are concentrated sulfuric acid and sulfur trioxide. When sulfur trioxide is dissolved in concentrated sulfuric acid, it forms oleum.

3. **Analyzing the Options**:
- **Option A: Castor Oil** - This is a type of vegetable oil and is not related to oleum.
- **Option B: Oil of Vitriol** - This term is often used to refer to sulfuric acid, but it does not specifically denote oleum.
- **Option C: Fuming H2SO4** - This is indeed another name for oleum, as it describes the fuming nature of the solution when SO3 is present.
- **Option D: None of them** - Since we have identified option C as correct, this option is incorrect.

4. **Conclusion**: The correct answer is **Option C: Fuming H2SO4**. Oleum is essentially a mixture of sulfur trioxide in concentrated sulfuric acid, which gives it the fuming characteristic.