Option 1 -> "Manipulated the throne" is not idiomatic and doesn't convey the act of seizing power.
Option 2 -> "Abandoned" means to give up or leave, which is opposite to taking control of the throne.
Option 3 -> "Attacked" could refer to a physical assault but doesn't specifically mean taking power illegally.
Option 4 -> "Usurped" means to take a position of power or authority illegally or by force, especially from the rightful holder.
Hence, Option 4: usurped -> "Usurp" is the precise term for illegally seizing power or a throne from its rightful ruler. In this context, the younger son wrongfully took the throne from his father (the legitimate king) and forced him into exile, which is the definition of usurpation -> correct