**Core Concept:** In human embryology, the term "embryo" is used until a specific stage after which it is considered as a fetus.

**Why the Correct Answer is Right:** The correct answer refers to the stage when the fetus begins to develop external genitalia and the placenta becomes functional, as these are the defining characteristics that separate the embryo stage from the fetal stage.

**Why Each Wrong Option is Incorrect:**
A. This option is incorrect because the term "embryo" is used even after the development of external genitalia, as it is the stage of development before the fetus becomes evident externally.
B. This option is incorrect because, although the placenta is involved in fetal development, the term "embryo" is used until the appearance of external genitalia.
C. This option is incorrect because, while the placenta is essential for fetal development, the term "embryo" continues to be used until the development of external genitalia.
D. This option is incorrect because the term "embryo" is used until the development of external genitalia, which marks the transition to the fetal stage.

**Clinical Pearl / High-Yield Fact:** A key point to remember is that the term "embryo" is used up to the twentieth week of pregnancy (approximately 4 months), after which the fetus is referred to as a "fetus." This distinction is important for understanding the developmental stages and terminology in obstetric and neonatal care.

**Correct Answer:** .