Assertion : S.I. units are logical and coherent .
Reason : S.I. system of units is a rationalised system.

Options

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

Explanation

To solve the question regarding the assertion and reason about S.I. units, we can break it down step by step.

### Step-by-Step Solution

**Step 1: Understand the Assertion**
- The assertion states that "S.I. units are logical and coherent."
- Logical means that the units are based on clear and rational principles.
- Coherent means that the units are interrelated in a way that allows for straightforward calculations without the need for additional conversion factors.

**Step 2: Understand the Reason**
- The reason states that "S.I. system of units is a rationalised system."
- A rationalized system means that the units are defined in such a way that they are consistent and unified. For example, the unit of energy (Joule) is the same across different forms of energy (mechanical, thermal, etc.).

**Step 3: Analyze Coherence in S.I. Units**
- Coherence in S.I. units means that derived units can be expressed as products or quotients of base units without introducing numerical factors.
- For example, the unit of force (Newton) is derived from mass (kilogram) and acceleration (meter per second squared) without needing any additional conversion factors.

**Step 4: Analyze Rationalization in S.I. Units**
- The S.I. system is rationalized as it provides a unified approach to measuring different forms of energy using the same unit (Joule), unlike other systems like CGS where different units are used for different types of energy.

**Step 5: Conclusion**
- Both the assertion and reason are correct statements. However, the reason does not directly explain the assertion about coherence.
- Therefore, the final conclusion is that the assertion is true, the reason is true, but the reason is not the correct explanation of the assertion.

### Final Answer
- Assertion: True
- Reason: True
- Conclusion: Reason is not the correct explanation of the assertion.