The pair of elements which form ionic bond is .

Options

A. `C +Cl`
B. `H +F`
C. `Na + Br`
D. `O +H`

Explanation

To determine which pair of elements forms an ionic bond, we need to analyze the properties of the elements involved. Ionic bonds are typically formed between metals and non-metals, where there is a transfer of electrons from the metal (which has a low ionization energy) to the non-metal (which has a high electron affinity).

### Step-by-Step Solution:

1. **Identify the Elements**: Start by identifying the pairs of elements given in the question. Let's assume the pairs are (A) Na and Cl, (B) Mg and O, (C) C and Cl, and (D) H and F.

2. **Determine Electropositivity and Electronegativities**:
- Metals tend to lose electrons easily (electropositive), while non-metals tend to gain electrons (electronegative).
- For example, Sodium (Na) and Magnesium (Mg) are metals, while Chlorine (Cl) and Oxygen (O) are non-metals.

3. **Evaluate Ionization Energy and Electron Affinity**:
- **Ionization Energy**: The energy required to remove an electron from an atom. Metals have low ionization energies.
- **Electron Affinity**: The energy change when an electron is added to an atom. Non-metals have high electron affinities.
- In the pairs:
- Na has low ionization energy and Cl has high electron affinity.
- Mg has low ionization energy and O has high electron affinity.
- C has high ionization energy and Cl has high electron affinity (not favorable for ionic bond).
- H has moderate ionization energy and F has high electron affinity.

4. **Analyze the Pairs**:
- **Na and Cl**: Na can lose one electron easily, and Cl can gain one electron easily. This pair can form an ionic bond.
- **Mg and O**: Mg can lose two electrons, and O can gain two electrons. This pair can also form an ionic bond.
- **C and Cl**: Carbon does not easily lose its four valence electrons, making this pair unlikely to form an ionic bond.
- **H and F**: Hydrogen can lose one electron, and F can gain one electron. This pair can also form an ionic bond.

5. **Conclusion**: The pairs that can form ionic bonds are Na and Cl, Mg and O, and H and F. However, the most classic example of an ionic bond is between Na and Cl.

### Final Answer:
The pair of elements which form an ionic bond is **Na and Cl**.