Assertion. Diamond is the hardest possible substance and is a network covalent solid.
Reason. All the C atoms in diamond are `sp^(3)` hybridised.

Options

A. If both assertion and reason are true, and reason is the true explanation of the assertion.
B. If both assertion and reason are true, but reason not is the true explanation of the assertion.
C. If assertion is true, but reason is false.
D. If both assertion and reason are false.

Answer

To solve the question regarding the assertion and reason about diamond, we can break it down into clear steps:

### Step 1: Understand the Assertion
The assertion states that "Diamond is the hardest possible substance and is a network covalent solid."
- **Explanation**: Diamond is indeed known for its exceptional hardness, which makes it one of the hardest known materials. It is classified as a network covalent solid because it consists of a three-dimensional network of carbon atoms bonded together by strong covalent bonds.

### Step 2: Understand the Reason
The reason states that "All the C atoms in diamond are sp³ hybridized."
- **Explanation**: In diamond, each carbon atom undergoes sp³ hybridization, forming four equivalent sp³ hybrid orbitals. Each of these orbitals overlaps with the orbitals of neighboring carbon atoms to form strong covalent bonds, resulting in a rigid three-dimensional structure.

### Step 3: Analyze the Relationship Between Assertion and Reason
- Both the assertion and the reason are true statements. The hardness of diamond is a direct consequence of its strong covalent bonding and network structure, which is facilitated by the sp³ hybridization of carbon atoms.

### Step 4: Conclusion
Since both the assertion and the reason are true, and the reason provides a valid explanation for the assertion, we conclude that:
- **Final Answer**: Both the assertion and reason are true, and the reason is the correct explanation of the assertion.