The correct answer is D) He chooses Lyndon B. Johnson as his running mate.
The move that Kennedy used to help assure the Southern vote was to choose Lyndon B. Johnson as his running mate.
In the 1960 election, Democratic candidate John F. Kennedy and the Democratic Party made a good decision in naming Lyndon B. Jhonson as Vice President. With this decision, Kennedy assured the Southern vote because Jhonson was a Senator from Texas. Johnson also was the President of the Senate at the time and finished second in the ballot for the presidency in the Democratic election.