Concept:
Abortion law deals with medical, ethical and social issues.
In India, the abortion limit was extended under legal conditions for certain categories.
Step 1: Check Assertion (A).
The assertion says that the legal abortion limit in India has been extended from 20 to 24 weeks.
This is correct under the amended legal framework for specific cases.
Step 2: Check Reason (R).
The reason says that the birth of unwanted children can add to over-population.
This statement may be considered generally correct as a social concern.
Step 3: Check explanation.
However, the legal extension is mainly related to reproductive rights, medical necessity, fetal abnormalities, and protection of vulnerable women.
It is not primarily explained by the over-population argument.
Step 4: Final conclusion.
Both statements are correct, but Reason is not the correct explanation of Assertion.
Hence:
\[
\boxed{\text{(B)}}
\]