The correct answer is C; Khrushchev treated Kennedy badly and disrespectfully. The Vienna conference occurred shortly after the failed Bay of Pigs invasion of Cuba which had made the U.S. look incompetent. Khrushchev knew that Kennedy was young and new to the office, and gave Kennedy the same treatment he had attempted to give President Dwight Eisenhower at the failed Paris summit several years earlier. Khrushchev had attempted to browbeat both Eisenhower and later Kennedy. Eisenhower had stood his ground; but Kennedy did not do so; in fact his only response was to call up reserve military units in the U.S. to demonstrate that he meant business. This did not stop Khrushchev from erecting the Berlin Wall, which he had threatened to do at the Vienna conference. Kennedy did not respond to Khrushchev's actions, which led Khrushchev to move forward with plans to place Soviet missiles in Cuba.
The Berlin Wall and missiles in Cuba were a result of Kennedy's failure to respond decisively to Khruschev's browbeating at Vienna. Had he been more experienced, and perhaps shown more backbone, the latter events may not have happened.