**Core Concept**
Preeclampsia is a pregnancy complication characterized by high blood pressure and signs of damage to another organ system, most often the liver and kidneys. Expectant management of preeclampsia involves closely monitoring the condition without immediate delivery, usually until the pregnancy reaches a certain gestational age or until the mother's condition worsens.

**Why the Correct Answer is Right**
The most common complication of expectant management of preeclampsia is placental abruption. This occurs when the placenta separates from the uterus, leading to severe bleeding and potentially life-threatening complications for both the mother and the fetus. Placental abruption is more likely to occur in women with preeclampsia due to the underlying vascular damage and increased blood pressure. The risk of placental abruption increases with gestational age, making close monitoring essential.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fetal growth restriction is a possible complication of preeclampsia, but it is not the most common complication of expectant management. Fetal growth restriction occurs due to decreased placental perfusion and is often seen in women with severe preeclampsia.
* **Option B:** Eclampsia, or the onset of seizures in a woman with preeclampsia, is a serious complication but not the most common one due to expectant management. Eclampsia usually occurs in women with severe preeclampsia who are not receiving adequate treatment.
* **Option C:** HELLP syndrome, which stands for Hemolysis, Elevated Liver enzymes, and Low Platelet count, is a severe complication of preeclampsia. However, it is not the most common complication of expectant management, and its onset is often unpredictable.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that expectant management of preeclampsia requires close monitoring, including regular blood pressure checks, fetal ultrasound, and laboratory tests to assess liver and kidney function. Early recognition of complications, such as placental abruption, is crucial to prevent severe maternal and fetal morbidity.

**Correct Answer:** C. Placental abruption.