Final answer:
Magnesium sulfate is the drug used to prevent seizures in preeclampsia, and it works by stabilizing neurons, not by affecting the loop of Henle-like diuretics.
Step-by-step explanation:
The drug used to reduce central nervous system (CNS) irritability and prevent seizures in patients with preeclampsia is B) Magnesium sulfate. This agent is not an oliguric diuretic and does not work on the loop of Henle to inhibit the reabsorption of Na+ and Cl-. Instead, magnesium sulfate is thought to stabilize neurons and prevent excitability that can lead to seizures, which is a complication of preeclampsia known as eclampsia. The other options listed, such as oxytocin, methyldopa, and nifedipine, serve different purposes: oxytocin is used to induce labor, methyldopa is an antihypertensive commonly used in pregnancy, and nifedipine is a calcium channel blocker also used for hypertension but not for the prevention of seizures.