The pair that requires calcination is

Options

A. `ZnCO_3 and CaO`
B. `Fe_2O_3 .xH_2Oand CaCO_3.MgCO_3`
C. `ZnO and Fe_2O_3.xH_2O`
D. All of these

Answer

To determine the pair that requires calcination, we need to understand what calcination is and which types of ores typically undergo this process.

### Step-by-Step Solution:

1. **Definition of Calcination**: Calcination is the process of heating an ore in the absence or limited supply of oxygen. This is typically done to remove volatile impurities, moisture, or to decompose carbonate ores.

2. **Identify the Types of Ores**: The question provides several pairs of substances:
- Zinc carbonate and calcium oxide
- Hydrated iron oxide and calcium carbonate
- Magnesium carbonate and zinc oxide
- Hydrated iron oxide and hydrated zinc oxide

3. **Recognize Carbonate Ores**: Calcination is particularly relevant for carbonate ores. When heated, these ores decompose to form oxides and release carbon dioxide gas. For example:
- Zinc carbonate (ZnCO3) decomposes to zinc oxide (ZnO) and carbon dioxide (CO2).
- Magnesium carbonate (MgCO3) decomposes to magnesium oxide (MgO) and carbon dioxide (CO2).

4. **Identify Moisture-Containing Ores**: Ores that contain water of crystallization or moisture also undergo calcination to remove these components. For example:
- Hydrated iron oxide (Fe2O3·nH2O) will lose water upon heating.

5. **Analyze the Options**:
- **Zinc carbonate and calcium oxide**: Zinc carbonate is a carbonate ore and would require calcination.
- **Hydrated iron oxide and calcium carbonate**: Hydrated iron oxide requires calcination due to moisture, and calcium carbonate also decomposes.
- **Magnesium carbonate and zinc oxide**: Magnesium carbonate is a carbonate ore and requires calcination.
- **Hydrated iron oxide and hydrated zinc oxide**: Hydrated iron oxide requires calcination due to moisture.

6. **Conclusion**: Among the pairs, the one that specifically requires calcination due to the presence of a carbonate ore is **Zinc carbonate**. Therefore, the correct answer is the pair that includes a carbonate ore.

### Final Answer:
The pair that requires calcination is **Zinc carbonate and calcium oxide**.