Final answer:
The correct adjective for a lively or animated disposition is 'boisterous', whereas 'gloomy' best describes the afterlife in the Sumerian belief system, reflecting a bleak and desolate underworld.
Step-by-step explanation:
The 9-letter adjective that means lively or animated is boisterous. However, to accurately answer the question about Sumerian belief regarding the afterlife, the adjective that best describes the afterlife according to the Sumerian belief system is gloomy.
According to Sumerian mythology, the afterlife was viewed as a bleak underworld known as the 'House of Dust', where souls resided in darkness, feeding on nothing but dust. This reflects a rather desolate and dreary outlook that is better captured by the term gloomy, indicating a somber or depressing character.