Final answer:
The President can send the National Guard to enforce desegregation in the South, as this falls under the purview of the executive branch and the President's role as Commander-in-Chief.
Step-by-step explanation:
In the 1950s and 1960s, the President, as the head of the executive branch, had the ability to take certain actions to address civil rights issues. Among the options provided, the action that the President could take is to send the National Guard to enforce desegregation in the South. This is because the President is the Commander-in-Chief of the armed forces and has the authority to deploy troops domestically to enforce federal laws and Supreme Court decisions. This action was apparent during the Little Rock Nine incident in 1957, when President Dwight D. Eisenhower sent federal troops to ensure the safe entry of African American students into a previously segregated high school in Arkansas. The other options listed, such as amending the constitution, applying the due process clause, or creating a law, are functions of the legislative and judicial branches, respectively.