**Core Concept**
After fertilization in the uterine tube, the zygote undergoes cleavage, morula formation, and eventually becomes a blastocyst. This process typically takes about 4-5 days, after which the blastocyst prepares for implantation in the endometrium.

**Why the Correct Answer is Right**
The blastocyst's transition to implantation is facilitated by the production of human chorionic gonadotropin (hCG) and other hormones that help in the preparation of the endometrium. By day 5, the blastocyst has reached the uterine cavity and is ready to initiate implantation. This process involves the trophoblast cells invading the endometrial lining, leading to the formation of the placenta. The exact timing can vary, but generally, implantation begins around day 5-6 post-fertilization.

**Why Each Wrong Option is Incorrect**
**Option A:** Implantation cannot occur within minutes because the blastocyst needs time to develop and prepare the endometrium for invasion.
**Option B:** By 12 hours, the zygote has just begun to cleave and form a morula, which is not yet ready for implantation.
**Option C:** By day 1, the zygote has not yet undergone the necessary transformations to become a blastocyst, and implantation is not imminent.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that implantation is a complex process requiring precise timing and coordination between the blastocyst and the endometrium. Any disruptions in this process can lead to implantation failures or abnormalities.

**✓ Correct Answer: D. By day 5**