**Core Concept**
Implantation is a critical process in early pregnancy where the blastocyst attaches to the uterine wall. This process is facilitated by the **trophoblast** cells of the blastocyst and the **endometrium** of the uterus.

**Why the Correct Answer is Right**
The correct timing of implantation is around 6-10 days post-fertilization, when the blastocyst reaches the uterine cavity. This timing allows for the optimal development of the embryo and the preparation of the uterine lining for implantation. The **human chorionic gonadotropin (hCG)** produced by the trophoblast cells plays a crucial role in maintaining the corpus luteum and supporting early pregnancy.

**Why Each Wrong Option is Incorrect**
**Option A:** Implantation does not occur immediately after fertilization, as the blastocyst needs time to develop and travel to the uterine cavity.
**Option B:** Implantation is not delayed until after the formation of the embryoblast, as the trophoblast cells are responsible for initiating the implantation process.
**Option D:** Implantation does not occur later than 10 days post-fertilization, as this would increase the risk of implantation failure and early pregnancy loss.

**Clinical Pearl / High-Yield Fact**
Implantation is a critical step in establishing a successful pregnancy, and any disruptions to this process can lead to early pregnancy complications.

**Correct Answer:** D. 6-10 days post-fertilization.