**Core Concept**
Reinke crystalloids are a characteristic histopathological feature of certain sex cord-stromal tumors of the ovary. They are needle-like or rod-shaped structures composed of hyaluronic acid and collagen, which are produced by the tumor cells.

**Why the Correct Answer is Right**
Reinke crystalloids are specifically associated with granulosa cell tumors, a type of sex cord-stromal tumor. The tumor cells in granulosa cell tumors produce and accumulate these crystalloids, which can be seen on histological examination. The presence of Reinke crystalloids is a diagnostic feature that helps differentiate granulosa cell tumors from other types of sex cord-stromal tumors.

**Why Each Wrong Option is Incorrect**
**Option A:** Sertoli-Leydig cell tumors are a type of sex cord-stromal tumor, but they are not typically associated with Reinke crystalloids.
**Option B:** Thecoma is a type of sex cord-stromal tumor, but it is not characterized by the presence of Reinke crystalloids.
**Option C:** Fibromas are a type of sex cord-stromal tumor, but they are not associated with Reinke crystalloids.

**Clinical Pearl / High-Yield Fact**
Reinke crystalloids are a distinctive feature of granulosa cell tumors, and their presence can help diagnose this type of tumor. It's essential to recognize this feature on histological examination to make an accurate diagnosis.

**Correct Answer:** C.