Final answer:
The technique of shaving the sides of the neck and across the nape with a razor is called a neck shave, which is different from a taper or a fade.
Step-by-step explanation:
The correct option that best completes the statement about the technique of shaving the sides of the neck and across the nape with a razor in the field of barbering or grooming is C) Neck shave. A neck shave is a technique used to clean up the hairline around the nape and sides of the neck, often with a straight razor or a safety razor, to provide a polished look. In contrast, a taper refers to the gradual decrease of hair length with clippers, usually around the ear line and back of the head. A fade is a more dramatic version of a taper where the hair progressively gets shorter down the sides and back to the skin. A nape shave could be seen as a variation or part of the term neck shave, but it is not often used as a standardized term in professional barbering.