Inert is best defined as...

• A. unable to move or act
• B. loud and energetic
• C. related to metal
• D. not understood

Answer: (A)

Inert describes something that is not moving or acting; it’s passive or motionless. That sense fits best because inert objects don’t initiate action or motion, and a person described as inert is seen as not taking action. The other options convey different ideas: loud and energetic is the opposite of inert, related to metal isn’t connected to the meaning, and not understood concerns comprehension rather than activity. (Note: in science, inert can also mean chemically nonreactive, which is still about a lack of activity, just in a chemical sense.)