Concept:
Citizenship determines the legal relationship between an individual and the state.
The Constitution of India provides rules regarding citizenship and related rights.
Step 1: Examine Assertion A.
India follows the principle of:
\[
\text{Single Citizenship}
\]
This means every Indian citizen holds citizenship of India only and not separate state citizenship.
Therefore, Assertion A is correct.
Step 2: Examine Reason R.
Under the Citizenship Act, a foreigner who is not an illegal migrant may acquire Indian citizenship through:
\[
\text{Naturalisation}
\]
subject to fulfillment of legal conditions.
Thus, Reason R is also correct.
Step 3: Check whether R explains A.
Reason R discusses one method of acquiring citizenship.
However, it does not explain why India follows single citizenship.
Hence, Reason R is not the correct explanation of Assertion A.
Step 4: Conclusion.
Therefore, the correct answer is:
\[
\boxed{\text{Both A and R are correct but R is NOT the correct explanation of A}}
\]