The word impromptu refers to something that is done without prior preparation or planning. It indicates that the speaker delivered their remarks spontaneously, without having planned or rehearsed them beforehand. Therefore, when looking for a synonym, we need a word that conveys this idea of unpreparedness.
Eloquently means speaking or writing in a fluent and persuasive manner, which does not necessarily imply lack of preparation.
Without preparation directly describes the essence of impromptu, indicating that the speaker did not prepare their remarks.
Without enthusiasm suggests a lack of excitement or interest, which is unrelated to the concept of being unprepared.
With great force indicates a strong manner of speaking but does not address preparation.
Thus, the meaning of the underlined word can best be captured by the phrase that signifies acting without preparation.