**Core Concept**
The morula is an early stage in the development of a mammalian embryo after the cleavage stage, and its implantation into the uterus is a critical step in establishing a successful pregnancy.

**Why the Correct Answer is Right**
The morula reaches the uterus around 3-4 days post-fertilization. As it travels through the fallopian tube, the morula undergoes compaction, where the cells start to stick together, forming a tightly packed ball of cells. This process is facilitated by the loss of the zona pellucida, a glycoprotein layer that surrounds the oocyte. Once the morula reaches the uterus, it undergoes further development and eventually forms a blastocyst, which is the stage that actually implants into the uterine lining.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is likely incorrect because it does not provide a specific time frame or developmental stage. The morula is a specific stage in embryonic development that occurs before implantation.

**Option B:** This option is likely incorrect because it does not accurately reflect the timing of morula implantation. The morula reaches the uterus before implantation occurs.

**Option C:** This option is likely incorrect because it is not a specific developmental stage or time frame. The morula is a distinct stage in embryonic development that occurs before implantation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the morula and blastocyst stages are critical for implantation, and any disruption during this period can lead to miscarriage or failed implantation.

**Correct Answer: C. 5-6 days post-fertilization**