**Core Concept**
Preeclampsia is a pregnancy complication characterized by new-onset hypertension (high blood pressure) and proteinuria (excess protein in the urine). The protein/creatinine ratio (P/C ratio) is a useful tool to diagnose proteinuria in pregnant women.

**Why the Correct Answer is Right**
A P/C ratio of 30 mg/mmol or greater is considered indicative of proteinuria and suggests the development of preeclampsia. This is because a P/C ratio of 30 mg/mmol or greater is associated with significant protein loss in the urine, which is a hallmark of preeclampsia. The kidneys' ability to filter waste and excess fluids is impaired in preeclampsia, leading to increased protein excretion.

**Why Each Wrong Option is Incorrect**
* **Option A:** A P/C ratio of less than 10 mg/mmol is generally considered normal and not indicative of proteinuria.
* **Option B:** A P/C ratio of 20-29 mg/mmol may be seen in mild proteinuria, but it is not diagnostic of preeclampsia.
* **Option C:** A P/C ratio of 40 mg/mmol or greater is more severe and indicative of significant kidney damage.

**Clinical Pearl / High-Yield Fact**
The American College of Obstetricians and Gynecologists (ACOG) recommends using the P/C ratio to diagnose proteinuria in pregnant women, with a threshold of 30 mg/mmol or greater indicating significant protein loss.

**Correct Answer:** C. 40 mg/mmol or greater.