The prince refused to commit regicide against his father. what does the root reg or rex mean? a. prince b. kill c. king d. father

The answer is letter B. The root word of reg- or rex- means "king". While the suffix word -cide indicates the act of killing.

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The definition of regicide is the act of killing a king
reg- means king
The broad definition of regicide is the deliberate killing of a monarch, or the person responsible for the killing of a person of royalty. In a narrower sense, in the British tradition, it refers to the judicial execution of a king after a trial.
B. kill