**Core Concept**
The endometrial biopsy is a diagnostic tool used to assess the endometrium's receptivity and histological changes in infertile women. It involves evaluating the endometrial lining for potential abnormalities that may affect implantation of the embryo.

**Why the Correct Answer is Right**
The optimal time for performing an endometrial biopsy in infertile women is during the mid-luteal phase, specifically between days 21 and 24 of a 28-day menstrual cycle. At this stage, the endometrium undergoes a window of receptivity for embryo implantation, and the biopsy can detect the presence of a secretory endometrium, which is crucial for implantation. This time frame allows for the assessment of progesterone-induced changes in the endometrium, which are essential for embryonic development.

**Why Each Wrong Option is Incorrect**
**Option A:** Performing a biopsy during the proliferative phase (days 1-14) may not accurately assess the endometrial receptivity, as it is primarily a proliferative phase with minimal progesterone effects.
**Option B:** A biopsy during the menstrual phase (days 1-5) would not provide any useful information about endometrial receptivity or histological changes.
**Option C:** A biopsy during the pre-ovulatory phase (days 14-18) may not capture the secretory changes induced by progesterone, which are crucial for implantation.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the timing of the endometrial biopsy is critical for accurate assessment of endometrial receptivity. A biopsy performed too early or too late may yield misleading results, potentially leading to incorrect diagnoses and unnecessary treatments.

**Correct Answer:** C.