John f. kennedy replaced eisenhower's concept of massive retaliation with a response strategy.

The right answer is "flexible" - a flexible response strategy.

The context for this question is the Cold War, and specifically, the strategy of effectively opposing the communist states and preventing any attack on the US. The term "massive retaliation" refered to the idea that the best way of deterring potential aggressors is by making it known that any agression would be met with a very powerful anwer, such as a nuclear attack.

John F. Kennedy didn't follow this strategy, and instead favoured "flexible response", which would not be limited to nuclear weapons, but could use also less powerfull weapons, if these would be adequate.