Inert

The most appropriate antonym of the word inert is active.

• Inert means "not moving or able to move; inactive."
• Active means "moving or capable of moving; having or showing energy or activity."

The other words do not have the same meaning as "inert" and are therefore not antonyms.

• Dormant means "inactive or resting, especially for a long period."
• Inoperative means "not working or functioning."
• Efficient means "able to do or produce a desired result with minimum wasted effort or expense."