Final answer:
Folic acid is the micronutrient that has been added to flour in the United States since the 1990s to help prevent birth defects and replace nutrients lost during processing.
Step-by-step explanation:
The micronutrient that has been added to flour in the United States since the 1990s is folic acid. The practice of adding folic acid to enriched flour aims to replace vitamins and minerals lost during processing, also known as enrichment, and to help prevent birth defects. Other nutrients commonly added during the enrichment process include iron, Vitamin B1 (thiamin), and niacin.