A monetary fee that citizens must pay to vote is called a poll tax.
The term that correctly fills in the blank is "poll tax." A poll tax is a monetary fee that citizens must pay in order to vote. These taxes were historically used in some countries as a means to disenfranchise certain groups, particularly minority and lower-income individuals. However, it is important to note that poll taxes have been largely abolished or deemed unconstitutional in many democracies today as they are considered a violation of the principle of equal suffrage.
