Final answer:
The correct 9-letter word for a procession of cars, particularly that of the U.S. president, is 'motorcade'.
Step-by-step explanation:
The 9-letter word that is often used to refer to a procession of cars, especially that of the U.S. president, is 'motorcade'. A motorcade is a procession or parade of vehicles, particularly one composed of limousines or other official cars in which VIPs or dignitaries travel in a ceremonial or formal event. When it comes to the president of the United States, a 'motorcade' typically includes armored vehicles and is accompanied by a significant security presence. The term 'motorcade' specifically fits the context given, while 'cavalcade', 'convoy', and 'parade' do not match the standard usage when referring to a presidential procession.