Which of the following is considered a root word?

A root word is the base part of a word that carries the primary meaning, often found in various forms and can be modified with prefixes and suffixes. In this context, "Poly" stands out as it derives from Greek, meaning "many." It serves as a foundational component in words such as "polymer" (many parts) and "polygamy" (many marriages), illustrating how it conveys a significant concept.

The other options provided are prefixes that modify root words rather than acting as root words themselves. "Un" expresses negation, as seen in "unhappy," "Re" indicates repetition or backward motion, as in "redo," and "Anti" signifies opposition, as in "antibiotic." While each of these prefixes is important in word formation, they do not function as root words that convey complete meanings on their own like "Poly" does.