What is the study of the structure and growth of the nail known as?

The study of the structure and growth of the nail is known as onychology. Onychology specifically focuses on the anatomy, physiology, and any pathologies related to the nails. This specialized field encompasses various aspects such as nail growth patterns, health, diseases, and different types of nails, providing vital knowledge that is essential for nail technicians to understand in order to care for and treat nails effectively.

While dermatology deals with the skin and its diseases, and cosmetology covers a broader range of beauty treatments including hair, makeup, and skin care, they do not focus specifically on nails. Manicuring refers to the cosmetic treatment of the hands and fingernails but does not involve the scientific study of nails, which is what onychology entails. This makes onychology the most relevant and accurate term for the study of nails among the choices presented.