The correct answer is choice A) Serum magnesium level. Magnesium sulfate is a mineral that reduces seizure risks in women with preeclampsia. It is often given intravenously and can also be used to prolong pregnancy for up to two days. However, magnesium sulfate can also cause side effects and toxicity, such as respiratory depression, muscle weakness, and cardiac arrest. Therefore, the nurse should monitor the serum magnesium level to ensure that it is within the therapeutic range of 4 to 7 mg/dL.

Choice B) Serum potassium level is wrong because magnesium sulfate does not affect the potassium level significantly.

Potassium is an electrolyte that regulates the nerve and muscle function, especially the heart.

The normal range of potassium level is 3.5 to 5.0 mEq/L.

Choice C) Serum sodium level is wrong because magnesium sulfate does not affect the sodium level significantly.

Sodium is another electrolyte that helps maintain fluid balance and blood pressure.

The normal range of sodium level is 135 to 145 mEq/L.

Choice D) Serum calcium level is wrong because magnesium sulfate does not affect the calcium level significantly.

Calcium is a mineral that is important for bone health, blood clotting, and muscle contraction.

The normal range of calcium level is 8.5 to 10.2 mg/dL.