Which of the following is not a substitutional solid?

Options

A. Brass
B. Brozne
C. steel
D. Monel metal

Explanation

To solve the question "Which of the following is not a substitutional solid?", we need to understand the concept of substitutional solids and identify the options provided in the question.

### Step-by-Step Solution:

1. **Understanding Substitutional Solids**:
- A substitutional solid is a type of solid solution where one type of atom can replace another type of atom in the crystal lattice. This typically occurs in alloys where the atoms of different metals can substitute for each other.

2. **Analyzing the Options**:
- We need to evaluate each option to determine if it is a substitutional solid or not.

3. **Option 1: Brass**:
- Brass is an alloy of copper and zinc. In brass, zinc atoms can replace copper atoms in the lattice structure. Therefore, brass is a substitutional solid.

4. **Option 2: Bronze**:
- Bronze is an alloy primarily made of copper and tin. Similar to brass, the tin atoms can replace some of the copper atoms in the lattice. Thus, bronze is also a substitutional solid.

5. **Option 3: Steel**:
- Steel is primarily an alloy of iron and carbon. In steel, iron occupies the lattice positions, while carbon atoms fit into the interstitial spaces (gaps) between the iron atoms. This means that steel does not involve the replacement of one atom by another in the same lattice position, making it an interstitial solid rather than a substitutional solid.

6. **Option 4: Monel Metal**:
- Monel metal is an alloy of nickel and copper. In this alloy, nickel atoms can replace copper atoms in the lattice structure, categorizing it as a substitutional solid.

7. **Conclusion**:
- Based on the analysis, the only option that is not a substitutional solid is **Option 3: Steel**.

### Final Answer:
**Steel is not a substitutional solid.**