The correct answer is A) It acts as a transition to the part of his speech in which he addresses his promises to the country and world.
The purpose of the following short sentence in President Kennedy’s speech is that It acts as a transition to the part of his speech in which he addresses his promises to the country and world.
When President Kennedy says "This much we pledge and more," he has expressed his promises, enumerate some and when heads "and more," he is implying the effort and commitment it is needed to get it done. That there are more things to considerate and with "more" these have to be considered.
b. The weather forecast calls for rain, we'll play outdoors anyway.
c. The weather forecast calls for rain, but we'll play outdoors anyway.
d. The weather forecast calls for rain but we'll play outdoors anyway.