**Core Concept**
The period of embryo is a critical stage in human development, spanning from fertilization to the end of the eighth week. During this period, the embryo undergoes significant morphological and histological changes, including gastrulation, neurulation, and organogenesis.

**Why the Correct Answer is Right**
The correct answer is **C. 8 weeks**. This is because the period of embryo is defined as the time from fertilization to the end of the eighth week, after which it is referred to as a fetus. The eighth week marks a significant milestone in embryonic development, as the major organs and body systems begin to take shape. The embryonic period is characterized by rapid growth and differentiation, with the formation of the neural tube, heart, lungs, and other vital organs.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the period of embryo does not begin at fertilization, but rather after the completion of implantation, which typically occurs around 6-7 days post-fertilization.
**Option B:** This is incorrect because the period of fetus begins after the eighth week, not before.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the period of embryo is a critical period of rapid growth and differentiation, and any disruptions during this time can have significant consequences for fetal development and long-term health outcomes.

**Correct Answer:** C. 8 weeks