A) he had represented hope for a better future.
B) no president had ever been assasinated before.
C) his domestic programs had stamped out poverty in America.
D) they believed only he could stop a communist takeover of Cuba.
Three of these options are completely false. The only possible answer is A. The main reason that people mourned JFK as that his administration was the time of "Camelot." JFK was an attractive, young man with a nice family. He seemed to be the symbol of a country on the rise, moving towards better times.
JFK was not the first president assassinated. Even if you don't remember that McKinley or Garfield was assassinated, you should remember that Lincoln was. After JFK was killed, Pres. Johnson launched the War on Poverty, so poverty obviously hadn't been stamped out. The communists had already taken Cuba before JFK ever became president. That is why he was involved in things like the Bay of Pigs invasion and the Cuban Missile Crisis.
A is the only possible answer.