The reactivities of `C Cl_(4)` and `SiCl_(4)` are different. For example `SiCl_(4)` can be easily hydrolysed and is prone to substitution reactions, whereas `C Cl_(4)` is inert.
The observed difference is because
(a)carbon atom has smaller size than hence substitution is not possible
(b)carbon is more electronegative than silicon
( c)silicon has low lying unoccupied orbitals
(d)`C-Cl` bond is stronger than `Si-Cl` bond

Answer

To solve the question regarding the differing reactivities of \( CCl_4 \) (carbon tetrachloride) and \( SiCl_4 \) (silicon tetrachloride), we can analyze the properties of these compounds and their respective bonds.

### Step-by-Step Solution:

1. **Understanding the Compounds**:
- \( CCl_4 \) is carbon tetrachloride, where carbon is bonded to four chlorine atoms.
- \( SiCl_4 \) is silicon tetrachloride, where silicon is bonded to four chlorine atoms.

2. **Reactivity Comparison**:
- \( SiCl_4 \) is known to undergo hydrolysis and substitution reactions readily.
- \( CCl_4 \) is largely inert and does not undergo these reactions.

3. **Analyzing Bond Strength**:
- The key difference in reactivity can be attributed to the strength of the \( C-Cl \) bond compared to the \( Si-Cl \) bond.
- The \( Si-Cl \) bond is weaker than the \( C-Cl \) bond, making it easier for \( SiCl_4 \) to react with water (hydrolysis) or other nucleophiles (substitution).

4. **Evaluating the Options**:
- (a) Carbon atom has a smaller size than silicon, hence substitution is not possible.
- This is incorrect; size does not directly influence reactivity in this context.
- (b) Carbon is more electronegative than silicon.
- While true, electronegativity does not explain the observed reactivity differences.
- (c) Silicon has low-lying unoccupied orbitals.
- This is not a necessary explanation for the reactivity of \( SiCl_4 \).
- (d) \( C-Cl \) bond is stronger than \( Si-Cl \) bond.
- This is the correct statement explaining why \( CCl_4 \) is inert while \( SiCl_4 \) is reactive.

5. **Conclusion**:
- The observed difference in reactivity between \( CCl_4 \) and \( SiCl_4 \) is primarily due to the strength of the \( C-Cl \) bond being stronger than the \( Si-Cl \) bond, making \( SiCl_4 \) more prone to hydrolysis and substitution reactions.

### Final Answer:
The correct answer is (d) \( C-Cl \) bond is stronger than \( Si-Cl \) bond.