Final answer:
Kennedy's repetition in his speech stresses 'Unity and common purpose' through the use of parallelism, emphasizing collaborative national efforts for liberty.
Step-by-step explanation:
The repetition of the wording "not to our ... but to our ..." in Kennedy's speech emphasizes the idea of Unity and common purpose. This rhetorical device is a form of parallelism, which is often used to structure an argument and create emphasis on the core message.
Kennedy's speech, through the use of parallelism, shows a commitment to working together for the common good, specifically referencing the nation's efforts to ensure the success of liberty. Such repeated structures help to underscore a collective national effort rather than divisions, thus highlighting A. Unity and common purpose as the key idea.