The correct answer is D) He sent troops to protect the students as they entered school.
The "Little Rock 9" situation refers to when 9 African American students tried to enter Central High School in Little Rock Arkansas after the Brown vs. Board of Education case ruled that "separate but equal" facilities for black and white students violated the 14th amendment of the US Constitution. When these students tried to enter the school on the first day, the governor of Arkansas called for the Arkansas guard to stop them from entering.
After hearing about this, Eisenhower sent the National Guard to ensure that this desegregation took place.