Final answer:
Acetaminophen is the recommended pain reliever for a pregnant woman at 20 weeks for headache relief, as it is safe for occasional use and has fewer side effects compared to NSAIDs like aspirin and naproxen.
Step-by-step explanation:
The safest and most recommended option for headache relief in a pregnant woman who is 20 weeks into her pregnancy is D. Acetaminophen. It is considered to be safe for occasional use during pregnancy and is effective for pain relief and fever reduction. Options such as Aspirin (A), Excedrin Migraine, which contains aspirin and caffeine, and Aleve (C), which is a brand name for naproxen, are generally not recommended during pregnancy due to the potential risks they pose to both the mother and the developing fetus. Acetaminophen has fewer side effects, such as gastric distress, compared to aspirin, and does not have the risks associated with non-steroidal anti-inflammatory drugs (NSAIDs).