Assertion: Chlorides of `Li, Be` and Mg are covalent in nature
Reason: `Li, Be` and Mg have large cationic size in the s-block elements

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion
C. If Assertion is True but the Reason is False
D. If both Assertion & Reason are False

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the chlorides of lithium (Li), beryllium (Be), and magnesium (Mg) are covalent in nature.
- This is true. Lithium chloride (LiCl), beryllium chloride (BeCl2), and magnesium chloride (MgCl2) do exhibit covalent character due to the nature of their bonding.

2. **Understanding the Reason**:
- The reason states that Li, Be, and Mg have large cationic sizes.
- This is incorrect. In fact, Li+, Be2+, and Mg2+ are relatively small cations. The size of the cation plays a crucial role in determining the nature of the bond.

3. **Covalent Character and Cation Size**:
- According to Fajans' rules, smaller cation size leads to a greater covalent character in ionic compounds.
- Since Li+, Be2+, and Mg2+ are small cations, their chlorides exhibit significant covalent character.

4. **Conclusion**:
- The assertion is true, while the reason is false. Therefore, the correct answer is that the assertion is true and the reason is false.

### Final Answer:
The correct option is C: Assertion is true, Reason is false.