What is the only complete fabula praetexta that has survived?

The only complete fabula praetexta that has survived is indeed "Octavia." A fabula praetexta is a type of Roman play that typically deals with historical or Roman subjects and often features characters of high social status, such as nobles or important political figures. "Octavia" is attributed to the playwright Seneca, and it focuses on the themes of power, betrayal, and the political dynamics of the time.

This play is particularly significant because it reflects the historical context of the Julio-Claudian dynasty, specifically the tumultuous relationship between Emperor Nero and his first wife, Octavia. The survival of "Octavia" provides a unique insight into Roman literature and drama, illustrating the stylistic features and thematic concerns of the time. It stands apart from the other titles listed, most of which are not complete or belong to different genres, thereby solidifying its uniqueness as the sole surviving example of this specific category of Roman drama.