**Core Concept**
The maturation index in gynecology refers to the percentage distribution of parabasal, intermediate, and superficial cells in a vaginal smear. This index is used to assess estrogen and progesterone levels in the body, particularly during the menstrual cycle. The mid-secretory phase is a critical period during the luteal phase when progesterone levels are high.

**Why the Correct Answer is Right**
During the mid-secretory phase, the high levels of progesterone cause an increase in the number of superficial cells in the vaginal smear. This is because progesterone stimulates the maturation of cells from the parabasal to the superficial stage. Superficial cells are larger and more mature, with a larger nucleus and more abundant cytoplasm. They are also more fragile and prone to lysis, which is why they are more abundant in the vaginal smear during this phase. The increased number of superficial cells is a reflection of the high progesterone levels in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because parabasal cells are the most immature cells in the vaginal smear and are typically seen in the proliferative phase of the menstrual cycle, when estrogen levels are high.

**Option B:** This option is incorrect because intermediate cells are a mix of mature and immature cells, and their percentage does not change significantly during the mid-secretory phase.

**Option C:** This option is incorrect because the maturation index in the mid-secretory phase is characterized by an increase in superficial cells, not a decrease.

**Clinical Pearl / High-Yield Fact**
The maturation index can be used to diagnose hormonal imbalances, such as anovulatory cycles or luteal phase defects, where the progesterone levels are low. A low percentage of superficial cells in the vaginal smear can indicate a luteal phase defect.

**Correct Answer: C. Superficial cells.**