The correct answer is B) the military.
The area of society that the federal government legally desegregate first was the military.
When we use the term "desegregate" in this question it means desegregation, the end of the separation between black and whites after the American Civil War. The first institution that ended segregation in the United States was the military. The Union army during the Civil War allowed African Americans to enroll in the Union army. Blacks saw the opportunity to get a better life after the war and joined the army in large numbers. Historians consider that approximately 180,000 African Americans enlisted.