## Core Concept
The question pertains to the legal definition of abortion, specifically focusing on the gestational age at which the termination of pregnancy is considered legally permissible. Abortion laws vary globally, but a common criterion is the viability of the fetus or a specific gestational age.

## Why the Correct Answer is Right
The correct answer hinges on understanding the legal frameworks surrounding abortion. The Medical Termination of Pregnancy (MTP) Act in many jurisdictions, including India, allows for the termination of pregnancy up to a certain gestational age under specific conditions. Typically, this is up to 20 weeks, but it can vary. The essence here is that the legal definition of abortion is closely tied to the concept of fetal viability and the gestational age limit set by law.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a gestational age, making it vague and not directly aligned with legal definitions that usually reference specific durations.
- **Option B:** This option suggests a gestational age that might not align with the legal limit in many jurisdictions, which often is set at 20 weeks.
- **Option C:** Similarly, this option might propose a gestational age that does not match the commonly accepted legal threshold for abortion.
- **Option D:** Without specific details on gestational age, it's hard to directly refute, but typically, legal abortion is defined by specific gestational age limits.

## Clinical Pearl / High-Yield Fact
A crucial point to remember is that the legal gestational age limit for abortion varies by jurisdiction. For example, under the MTP Act, in India, the limit is generally up to 20 weeks, but there's ongoing debate and variation in laws globally. Clinicians must be aware of the laws in their jurisdiction.

## Correct Answer: D. 20 weeks.