ব্যাখ্যা
The correct answer is option (d) "enemy."
'Ally' means a person, country, or organization that cooperates with or helps another; a friend or partner.
The antonym (opposite) of ally is 'enemy' — someone who is hostile or opposed.
Related words: Ally (noun/verb), Alliance (noun), Allied (adjective).
In international relations, allied nations fight together; enemy nations are opponents.
Source: Oxford English Dictionary.