Which root word signifies "written"?

The root word that signifies "written" is "gram-." This prefix derives from the Greek word "gramma," which means something written or a letter. In various contexts, it is often used in words related to writing or recording, such as "grammar," referring to the structure of written language, and "telegram," a message sent in writing.

The other root words do not relate to the concept of writing. "Raph-" refers to seam or suture and is often used in anatomical terms, while "hydro-" pertains to water, commonly found in words like "hydraulic" and "hydrology." Lastly, "gen-" signifies birth or origin, appearing in terms like "generate" and "genesis." Thus, "gram-" is specifically tied to the idea of writing, making it the correct choice.