Explanation:

A 'harbinger' is a person or thing that announces or signals the approach of another; a forerunner. Option (2) 'forerunner' is a direct synonym, meaning a person or thing that precedes the coming or development of someone or something else. Option (1) 'atrocious' means horrifyingly wicked or of very poor quality. Option (3) 'free' means not under the control of another. Option (4) 'understand' means to perceive the meaning of. These options are semantically unrelated to 'harbinger'.