Tengwar is the invented script used for the One Ring inscription in J.R.R. Tolkien's writings.
Tengwar, also called the Fëanorian characters, is a writing system created by the Elf Fëanor in Tolkien's fictional history. The inscription on the One Ring is written in Tengwar letters, although the language of the inscription is the Black Speech of Mordor. Its opening words are “Ash nazg durbatulûk,” translated as “One Ring to rule them all.”
This distinction matters because a script and a language are different things. Tengwar is the alphabetic writing system; the inscription's language is not Quenya or Sindarin. The letters can represent different languages, much as one real-world script can be used to write several languages.
Cirth is another Tolkien writing system, associated especially with Dwarves and later adapted by other peoples. It is therefore a plausible but incorrect answer for the Ring inscription. The visible characters on the Ring are Tengwar.