Final answer:
The adjective that best describes the afterlife according to the Sumerian belief system is gloomy. Sumerians viewed the afterlife as a dark and dusty realm, differing greatly from more positive afterlife views in other cultures.
Step-by-step explanation:
To answer the student's question about the Sumerian belief system and the description of the afterlife, the most appropriate adjective would be gloomy. The Sumerians, one of the earliest civilizations, had a rather pessimistic view of the afterlife. According to their mythology and literary texts such as The Epic of Gilgamesh, the realm of the dead was a dreary, dark place where spirits resided in a state of dust and darkness, regardless of their behavior in the mortal world.
This bleak depiction contrasts with more positive or joyful afterlife concepts seen in other ancient cultures. So, when selecting an adjective to best describe the afterlife as per Sumerian beliefs from the given options, gloomy (option a) is the accurate choice. It reflects the somber and desolate nature of the underworld, known as Kur, in Sumerian religion. Unlike a happy, exciting, or scary afterlife, the Sumerian afterlife was characterized by eternal rest in darkness, which lacks the positive or thrilling connotations the other adjectives might imply.