Assertion : The name of the complex `[Co(NH_(3))_(6)]Cl_(3)` is hexamminecobalt (III) chloride.
Reason. When a prefix like hexa ends in 'a' and the ligand like 'ammonia' starts with 'a', only one 'a' is retained.

Options

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

Explanation

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

### Step 1: Analyze the Assertion
The assertion states that the name of the complex \([Co(NH_3)_6]Cl_3\) is hexamminecobalt(III) chloride.

- The complex consists of cobalt (Co) and ammonia (NH₃) ligands.
- There are six ammonia ligands, which is indicated by the prefix "hexa."
- The oxidation state of cobalt in this complex needs to be determined. Since ammonia is a neutral ligand, the charge contributed by the three chloride ions (Cl⁻) must be considered.

### Step 2: Determine the Oxidation State of Cobalt
- The total charge contributed by the three chloride ions is \(-3\) (since each Cl has a \(-1\) charge).
- To balance this, cobalt must have a \(+3\) charge to make the overall charge of the complex neutral.

### Step 3: Naming the Complex
- The name of the complex is constructed by naming the ligands first, followed by the metal and its oxidation state in Roman numerals.
- The six ammonia ligands are named "hexammine" (the 'a' from hexa is retained because it precedes the 'a' in ammonia).
- Therefore, the complete name is hexamminecobalt(III) chloride.

### Conclusion for Assertion
The assertion is **true**: The name of the complex \([Co(NH_3)_6]Cl_3\) is indeed hexamminecobalt(III) chloride.

### Step 4: Analyze the Reason
The reason states that when a prefix like "hexa" ends in 'a' and the ligand starts with 'a', only one 'a' is retained.

- In this case, "hexa" ends with 'a' and "ammonia" starts with 'a'.
- According to the rules of naming coordination compounds, we do indeed drop one 'a' from "hexa" when combined with "ammonia," resulting in "hexammine."

### Conclusion for Reason
The reason is also **true**: The naming convention correctly applies here.

### Final Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion.