Graphic novels are defined by their use of sequential art, meaning they tell stories through a sequence of illustrations.
· Sequential art: Correct substitution; describes storytelling through a sequence of images.
· Existential art: Unrelated; refers to existential themes. "Existential art" might refer to artwork dealing with existential themes, such as the nature of existence, purpose, or human struggles. This term does not describe the medium or style of graphic novels, as the sentence is about the format rather than the themes.
· Sedimental art: Incorrect; related to sediments or deposits.
· Longitudinal art: Incorrect; refers to long-term studies or directions.
Thus, the correct substitution is Sequential art in a traditional.
Hence, option (a) is the answer.