Final answer:
In his speech, Kennedy uses figurative language, specifically personification, by referring to the space effort as being in its 'infancy', to emphasize its potential and early success.
Step-by-step explanation:
In President John F. Kennedy's "We Choose to Go to the Moon" speech, the rhetorical device used to support his argument is figurative language, specifically personification. Kennedy describes the space effort as being in its 'infancy,' suggesting that it is young and has much potential for growth, similar to a human infant. This figure of speech emphasizes the early stages of space exploration while highlighting its rapid progress and the opportunities it has fostered, such as new companies and jobs. The options 'antithesis', 'irony', and 'rhetorical question' are not applicable in this context as Kennedy is not contrasting two opposing ideas, saying the opposite of what he means, or posing a question to make a point.