Final answer:
The refractive index of the medium between the specimen and objective lens is represented by the symbol 'n', which is essential in determining how much the medium refracts light.
Step-by-step explanation:
The symbol that represents the refractive index of the medium present between the specimen and the objective lens is C) n. In the equation NA = n sin α, 'n' is used to denote the index of refraction, which is the measure of how much a medium can bend or refract light. The refractive index 'n' is always greater than or equal to 1, with larger values indicating stronger refraction capabilities of the medium. Refractive index is a crucial concept in understanding optics as it affects the resolution and the detail that can be observed through a lens.