**Core Concept**
Implantation is the process by which a fertilized ovum (blastocyst) attaches to the endometrium, the inner lining of the uterus. This process is crucial for establishing a successful pregnancy. The endometrium undergoes a series of changes in response to hormonal fluctuations, preparing it for implantation.

**Why the Correct Answer is Right**
Implantation typically occurs 6-10 days after fertilization, when the blastocyst reaches the uterine cavity. The blastocyst consists of an inner cell mass and a trophectoderm, which come into contact with the endometrial glands and stroma. The interaction between the blastocyst and the endometrium is mediated by various cytokines and growth factors, such as human chorionic gonadotropin (hCG) and leukemia inhibitory factor (LIF). These molecules promote implantation by stimulating cell proliferation, migration, and differentiation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because implantation does not occur during the follicular phase, when the endometrium is thin and not prepared for implantation.

**Option B:** This option is incorrect because implantation does not occur during the luteal phase alone; it is a specific process that occurs within a narrow time window.

**Option C:** This option is incorrect because implantation does not occur before the blastocyst reaches the uterine cavity.

**Clinical Pearl / High-Yield Fact**
It is essential to recognize that implantation is a dynamic process and can be influenced by various factors, including hormonal imbalances, uterine anomalies, and immunological disorders. A thorough understanding of implantation is crucial for diagnosing and managing early pregnancy complications.

**Correct Answer: C. When the blastocyst reaches the uterine cavity.**