Assertion:Binding energy per nucleon is nearly constant for element in the range A=30 to A=170 .
Reason : The nuclear force between two nucleons falls rapidly to zero as their distance is more than a few femtometres.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion .
B. If both assertion and reason are true but reason is not the correct explanation of 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: Understand the Assertion
The assertion states that the binding energy per nucleon is nearly constant for elements in the range of A = 30 to A = 170.

**Explanation**:
Binding energy per nucleon is a measure of how tightly the nucleons (protons and neutrons) are held together in a nucleus. For stable nuclei, this value tends to be relatively constant across a range of atomic mass numbers (A). In the specified range (A = 30 to A = 170), we observe that the binding energy per nucleon does not vary significantly, indicating a saturation effect in the nuclear force.

**Hint**: Look for trends in binding energy across different atomic mass numbers to support the assertion.

### Step 2: Understand the Reason
The reason given is that the nuclear force between two nucleons falls rapidly to zero as their distance exceeds a few femtometers.

**Explanation**:
The nuclear force is indeed a short-range force, meaning it is effective only at very small distances (on the order of a few femtometers). As nucleons are packed closely together within a nucleus, they experience strong attractive forces. However, as the distance between them increases, this force diminishes quickly. This characteristic leads to a saturation effect, where adding more nucleons does not significantly increase the binding energy per nucleon, thus explaining the assertion.

**Hint**: Consider the nature of nuclear forces and how they affect the stability of larger nuclei.

### Step 3: Evaluate the Relationship
Now we need to determine if the reason provided is a correct explanation for the assertion.

- Both the assertion and the reason are correct.
- The reason explains why the binding energy per nucleon remains nearly constant due to the saturation of nuclear forces.

### Conclusion
Since both the assertion and the reason are correct, and the reason correctly explains the assertion, the correct option is:

**Option 1**: Both assertion and reason are correct, and the reason is the correct explanation of the assertion.

### Final Answer
The correct answer is Option 1.