Final answer:
The spelling for the word root that refers to 'nails' in medical terms is 'onych/o'.
Step-by-step explanation:
The correct spelling for the word root that means 'nails' is c. onych/o. This prefix is used in medical terminology to refer to the nails, commonly found in words describing conditions, diseases, or anatomy related to the nails. For example, onychomycosis (a fungal infection of the nails) or onychophagia (nail biting).