**Core Concept**
Organogenesis is the critical process of formation and development of organs and tissues during embryogenesis. It involves the intricate and highly regulated interactions between various cell types, signaling pathways, and genetic factors to shape the complex structures of the human body.

**Why the Correct Answer is Right**
The period of organogenesis is maximally affected during the 3rd to 8th week of gestation, also known as the embryonic period. During this time, the embryo undergoes a series of rapid and coordinated morphogenetic events, including gastrulation, neurulation, and the formation of major organ systems such as the cardiovascular, respiratory, and gastrointestinal systems. The intricate processes of cell migration, differentiation, and apoptosis are precisely regulated by a complex interplay of genetic and environmental factors, including transcription factors, growth factors, and hormones.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because organogenesis is not maximally affected during the 1st week of gestation, which is primarily characterized by fertilization, cleavage, and compaction of the zygote.
* **Option B:** This option is incorrect because organogenesis is not maximally affected during the 9th to 12th week of gestation, which is primarily characterized by the differentiation and growth of the major organ systems.
* **Option C:** This option is incorrect because organogenesis is not maximally affected during the 13th to 16th week of gestation, which is primarily characterized by the refinement and maturation of the major organ systems.

**Clinical Pearl / High-Yield Fact**
A key concept to remember is that the period of organogenesis is a critical window of susceptibility to teratogenic insults, including environmental toxins, radiation, and infections. Therefore, it is essential to avoid exposure to potential teratogens during this period to minimize the risk of congenital anomalies.

**Correct Answer: D. 3rd to 8th week of gestation**