Which of the following is not an alloy ?

Options

A. Brass
B. Bronze
C. Magnalium
D. Magnesium

Explanation

To determine which of the following is not an alloy, we need to understand what an alloy is. An alloy is a homogeneous mixture of two or more metals, or a metal and a non-metal. Let's analyze the options provided:

1. **Brass**: This is an alloy made from copper and zinc. Therefore, brass is an alloy.

2. **Bronze**: This is an alloy made from copper and tin. Thus, bronze is also an alloy.

3. **Magna-alium**: This is an alloy made from magnesium and aluminum. Hence, magna-alium is an alloy as well.

4. **Magnesium**: This is a pure element and not a mixture of metals or a metal and a non-metal. Therefore, magnesium is not an alloy.

Based on this analysis, the answer to the question "Which of the following is not an alloy?" is **magnesium**.

### Step-by-Step Solution:
1. **Define an Alloy**: An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal.

2. **Analyze the Options**:
- **Brass**: Copper + Zinc = Alloy
- **Bronze**: Copper + Tin = Alloy
- **Magna-alium**: Magnesium + Aluminum = Alloy
- **Magnesium**: Pure metal = Not an alloy

3. **Identify the Non-Alloy**: From the analysis, magnesium is the only option that is not an alloy.

4. **Conclusion**: The answer is magnesium.