Cementite is:

Options

A. interstitial compound of iron and carbon
B. an alloy of `Fe` and `Cr`
C. a compound resembling cement
D. an ore of iron

Answer

To determine what cementite is, we need to analyze the options provided and understand the definition of cementite.

### Step-by-Step Solution:

1. **Understanding Cementite**: Cementite is a compound that is formed from iron and carbon. It is important to know that it is not just any mixture or alloy, but a specific type of compound.

2. **Analyzing the Options**:
- **Option A**: Interstitial compounds of iron and carbon.
- **Option B**: An alloy of Fe and Cr (iron and chromium).
- **Option C**: A compound resembling cement.
- **Option D**: An ore of iron.

3. **Identifying the Correct Option**:
- **Option A** is correct because cementite (Fe3C) is indeed an interstitial compound where carbon atoms are trapped within the iron lattice.
- **Option B** is incorrect because cementite does not involve chromium.
- **Option C** is misleading; while it may resemble cement in some physical properties, it is not a compound that is defined as resembling cement.
- **Option D** is incorrect as cementite is not an ore but a compound formed from iron and carbon.

4. **Conclusion**: Based on the analysis, the correct answer is **Option A: Interstitial compounds of iron and carbon**.