**Core Concept**
The morula stage is a critical phase in embryonic development where the compacted cells undergo several key changes, ultimately leading to the formation of a blastocyst. This stage is characterized by the transformation of the early blastomeres into a compact, spherical mass of cells.

**Why the Correct Answer is Right**
The morula reaches the uterus approximately 3-4 days post-fertilization. At this stage, the zygote undergoes several cleavage divisions, resulting in a compact cluster of cells called the morula. The morula is then transported through the oviduct (fallopian tube) and reaches the uterine cavity, where it undergoes implantation. The implantation process involves the morula's transformation into a blastocyst, which then adheres to the uterine lining, initiating the development of a new individual.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as the morula stage occurs before the blastocyst stage, which is not correct.
* **Option B:** This option is incorrect as it fails to specify the correct timing of the morula's arrival in the uterus.
* **Option C:** This option is incorrect as it refers to a later stage of embryonic development, which is not relevant to the morula's arrival in the uterus.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the morula's arrival in the uterus marks the beginning of the implantation process, which is a critical phase in human development.

**Correct Answer: D. Approximately 3-4 days post-fertilization.**