Final answer:
A Usurper is the term for someone who unlawfully takes a throne from its rightful ruler.
Step-by-step explanation:
The correct term for someone who steals a throne from its rightful ruler is a Usurper. This term, usurper, refers to a person who unlawfully seizes and holds the position of an existing sovereign. For instance, during the Middle Ages, there were instances of usurpers seizing thrones through force or political maneuvering. While a Pretender claims the throne without rightful ownership, a Monarch is the reigning king or queen, and a Regent is someone who governs because the rightful monarch is unable to (due to age, incapacity, or absence).
Learn more about Usurper