**Core Concept**
Ovulation is a critical event in the menstrual cycle where the mature ovum is released from the dominant follicle in the ovary. Identifying signs of ovulation is essential for diagnosing various gynecological conditions. The sudden reduction in size of the follicle and the presence of free fluid in the Pouch of Douglas on ultrasound (USG) are indicative of ovulation.

**Why the Correct Answer is Right**
The sudden reduction in size of the follicle occurs due to the collapse of the follicular wall after ovulation. This collapse leads to the release of the ovum into the peritoneal cavity. The free fluid in the Pouch of Douglas on USG is a result of the spillage of follicular fluid into the peritoneal cavity. This finding is suggestive of ovulation because it indicates the release of the ovum from the follicle.

**Why Each Wrong Option is Incorrect**
**Option A:** Pseudostratification in the endometrium is a feature of the proliferative phase, not ovulation. It is characterized by the presence of ciliated and non-ciliated columnar epithelial cells.

**Option B:** The Spinnbarkeit phenomenon in cervical mucus is a feature of the luteal phase, not ovulation. It is characterized by the ability of the cervical mucus to be stretched like elastic, which is due to the high water content and the presence of progesterone.

**Option D:** A trilaminar endometrium on USG is a feature of the early secretory phase, not ovulation. It is characterized by the presence of three distinct layers in the endometrium, which is due to the effect of progesterone on the endometrium.

**Clinical Pearl / High-Yield Fact**
In clinical practice, a sudden reduction in size of the follicle and the presence of free fluid in the Pouch of Douglas on USG are often referred to as the "ovulatory sign". This sign is essential for diagnosing conditions such as ectopic pregnancy, where the ovum implants outside the uterus.

**✓ Correct Answer: C. Sudden reduction in size of follicle and free fluid in Pouch of Douglas on USG**