What type of hair accessory is primarily designed to replace existing hair?

The type of hair accessory primarily designed to replace existing hair is a wig. Wigs are crafted from either natural or synthetic hair and are intended to cover the entire scalp, effectively providing a complete new hairstyle or look for the wearer. They are frequently used for various reasons, including fashion, convenience, or to address hair loss issues.

In contrast, a bun is a hairstyle that involves gathering the hair into a knot or loop, utilizing the wearer's own hair rather than replacing it. A hat serves as a fashion accessory or protective gear but does not replace any part of the hair. Clip-in extensions are used to add length or volume to existing hair; they attach to the wearer’s natural hair instead of serving as a complete replacement. Thus, a wig is distinctly designed as a full coverage accessory, making it the correct choice for this question.