He abdicated his post of his own accord.

Answer: Option D

Abdicated : (of a monarch) renounce one's throne.
Acquired ; buy or obtain (an asset or object) for oneself.
Inherited : (of money, property, or a title) received as an heir at the death of the previous holder.
Liberate : set (someone) free from imprisonment, slavery, or oppression.
Usurped : take (a position of power or importance) illegally or by force.

Antonym of Abdicated is Usurped