Political machines in the 1800s were notorious for their intricate system of gaining and maintaining political power. Let's take a look at each option to better understand the main tactic they used.
A) Kickbacks: Although kickbacks were also used frequently in such political environments, they weren't the primary way for political machines to hold on to their power. Kickbacks are basically acts of misappropriation of funds returned to the benefactor as a result of the benefactor's altered valuation of an object of exchange.
B) Taxes: While the collected taxes might have further empowered them financially, their ability to remain in power was not mainly maintained through taxes. Governance is not merely about economic control; political influence is also crucial.
C) Bribes: Bribes, or illicit prepayments in return for a favor or service, could have definitely extended the tenure of some politicians in power. Still, it was not the main practice by political machines to keep their grip on power.
D) Votes: Political machines mostly traded money and favors for votes by manipulating voters or offering favors, funds, or influences for their votes in order to get reelected or remain in power.
Therefore, the correct answer is D) Votes. Votes were mostly traded for money and favors to secure elections and maintain control over the political landscape, a common practice used by political machines of the time.