Final answer:
The safe and effective treatment for pyelonephritis in a pregnant woman is Nitrofurantoin, as it has a favorable safety profile during pregnancy, unlike Ciprofloxacin and Trimethoprim-sulfamethoxazole, which can pose risks to the developing fetus.
Step-by-step explanation:
The optimal treatment for pyelonephritis in pregnant women should be both effective against the infection and safe for the developing fetus. Antibiotics such as nitrofurantoin and ampicillin are commonly used due to their safety profile during pregnancy. It is crucial to avoid medications that are contraindicated during pregnancy such as ciprofloxacin and trimethoprim-sulfamethoxazole, especially in the first trimester, as they can pose risks to the fetus. In this case, the preferred option would be B) Nitrofurantoin, which is commonly prescribed during pregnancy and shown to be safe for both the mother and the fetus.