**Core Concept**
The process of implantation of a fertilized ovum in the uterine endometrium involves the development of the embryo from a single cell to a multicellular blastocyst. This process is crucial for the establishment of pregnancy.

**Why the Correct Answer is Right**
By the time of implantation, the fertilized ovum has undergone several cell divisions, resulting in a blastocyst consisting of approximately 100-200 cells. This expansion is facilitated by the rapid cell division in the earliest stages of embryonic development. The blastocyst is composed of two main parts: the inner cell mass, which will give rise to the fetus, and the trophoblast, which will form the placenta.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it underestimates the number of cells present in a blastocyst at the time of implantation.
* **Option B:** This option is incorrect because it overestimates the number of cells present in a blastocyst at the time of implantation.
* **Option D:** This option is incorrect because it is not a commonly accepted estimate of the number of cells in a blastocyst at the time of implantation.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the timing and rate of embryonic development can be influenced by various factors, including maternal age, fertility treatments, and genetic abnormalities.

**Correct Answer: C. 100-200 cells**