Answer:
The answer to your question would be that the underlined word that functions as a preposition is of.
Explanation:
The only preposition in the sentence above is of. Machines is a noun, have is functioning as an auxiliary verb. Have + come and gone, behaving as the main verbs form the present perfect tense. Then, but is a conjunction, the is the definite determiner, spirit is a noun as well as invention. Finally, lives is a verb and forever an adverb.
