## Core Concept
The question tests the understanding of fetal development and radiosensitivity. Fetal radiosensitivity varies throughout gestation, with certain periods being more critical than others due to the rapid cell division and differentiation occurring during embryonic and fetal development.

## Why the Correct Answer is Right
The fetus is most radiosensitive between **8 to 15 weeks** of gestation. During this period, the fetus undergoes significant growth and development, including organogenesis. Ionizing radiation exposure during this critical period can lead to severe congenital anomalies and growth restriction. The period of **8 to 15 weeks** corresponds to a time when the major organs are forming, and the tissues are highly sensitive to radiation.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because the beginning of gestation (0-4 weeks) involves the initial stages of cell division and implantation, during which the embryo is also highly sensitive but the impact might not be as pronounced or manifest as it does in the organogenesis phase.
* **Option B:** This option suggests a broader and later period (16-20 weeks), which, although still a critical time for growth and development, is not considered the peak of radiosensitivity compared to the earlier period.
* **Option D:** This option suggests a very early period (0-4 weeks), which, although critical for implantation and initial development, does not represent the peak sensitivity related to organogenesis.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **8 to 15 weeks** gestational age period is crucial for fetal development, and exposure to ionizing radiation during this time can have severe consequences, including increased risk of congenital anomalies and childhood cancer. Pregnant women should avoid unnecessary exposure to radiation, and any necessary medical imaging should be carefully considered.

## Correct Answer: B.