Final answer:
A crack in a hand-held instrument is most likely to occur in areas that experience high stress or force during use, such as when the instrument is dropped or impacted.
Step-by-step explanation:
A crack in a hand-held instrument would most likely be found in the part that experiences the highest stress or force during use. In the given options, hitting a hard surface with a hammer (c) and blowing through a small slit in a stretched rubber band (d) do not involve a hand-held instrument. Therefore, they are not relevant to the question. A common scenario where a crack may occur in a hand-held instrument is when it is dropped or impacted. The impact can create stress points and lead to cracks in the instrument, especially in areas that are structurally weaker or experience significant forces.