You are correct to say that D is the best answer of those given here. B and C are, to some degree, matters of opinion, but most historians would say that these are both true. A is indisputably true as it is not a matter of opinion. Therefore, D is the best answer.
John F. Kennedy was not the youngest man ever to serve as president. That distinction belongs to Theodore Roosevelt. However, he was the youngest man ever to be elected president as Roosevelt first became president because he was the vice president when William McKinley was assassinated. Thus, A is correct.
Kennedy did clearly stand up to Khrushchev, with one such instance being the Cuban Missile Crisis. Whether America had lost its place in the world so that he needed to “reestablish” it is a matter of opinion, but it is a defensible position. Most historians would say that the decision to invade Cuba at the Bay of Pigs was a mistake either because it shouldn’t have been attempted at all or because it should have had more American support. Either way, it is typically seen as a mistake. For these reasons, B and C are also correct, making D the best answer.