💡 Detailed Explanation
The correct answer is '巧妙' (kōmyō), which means ingenious, clever, or skillful, often used to describe a mechanism, plan, or trick that is complex and well-made. '器用' (kiyō) describes a person who is skillful with their hands. '便利' (benri) means convenient. '利口' (rikō) means clever or smart, but it's typically used for people or animals, not for the structure of a machine.