**Core Concept**
Gastrulation is a fundamental process in embryonic development where the blastula undergoes a series of complex cellular movements and reorganizations to form the three primary germ layers: ectoderm, endoderm, and mesoderm. This process is crucial for the establishment of the basic body plan and organogenesis.

**Why the Correct Answer is Right**
Gastrulation typically occurs around 3 weeks after fertilization, during which time the embryo undergoes a series of coordinated cellular movements, including epiboly, invagination, and ingression. These movements result in the formation of the blastocoel, which eventually gives rise to the three primary germ layers. The correct timing of gastrulation is essential for the proper development of the embryo, and any disruptions can lead to severe developmental abnormalities.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrulation does not occur during the first few days after fertilization, as the embryo is still in the cleavage stage.
**Option B:** Gastrulation is not a process that occurs during the later stages of embryonic development, such as organogenesis or fetal development.
**Option C:** Gastrulation does not occur during the implantation stage, which occurs around 6-7 days after fertilization.

**Clinical Pearl / High-Yield Fact**
Gastrulation is a critical period in embryonic development, and any disruptions during this time can lead to severe developmental abnormalities, including neural tube defects, heart defects, and limb abnormalities.

**Correct Answer:** C. 3 weeks