The word "agony" is derived from the combining form "agon/o," which refers to pain or suffering. This is evident in the word "agony" itself, as it represents intense physical or mental pain. The other options, "fibro/o" and "a band," do not relate to the concept of agony. Similarly, while "arm" is a body part, it does not directly connect to the meaning of agony. Therefore, "agony" is the correct answer based on its association with the combining form "agon/o."