**Core Concept**
The **CA 125** tumor marker is primarily used in the monitoring and follow-up of patients with **ovarian cancer**. It is a mucin-like glycoprotein present on the surface of many ovarian cancer cells. The upper limit of normal for CA 125 is crucial for distinguishing between benign and malignant conditions.

**Why the Correct Answer is Right**
The correct upper limit of normal for CA 125 is **35 U/mL**. This value is significant because levels above 35 U/mL may indicate ovarian cancer, although elevated levels can also be seen in various benign conditions such as endometriosis, pregnancy, and pelvic inflammatory disease. The **assay sensitivity and specificity** are critical in determining this cutoff value.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it is too low, potentially leading to false negatives.
**Option B:** Incorrect as it is not the established cutoff.
**Option D:** Incorrect because it is higher than the established normal limit.

**Clinical Pearl / High-Yield Fact**
Remember, a single elevated CA 125 level is not diagnostic of ovarian cancer; **serial measurements and correlation with clinical findings** are essential.

**Correct Answer:** C. 35 U/mL