Final answer:
The god who threatened to kill Loki at Frey's feast for his betrayal in Norse mythology was Heimdall. This incident occurred during Ægir's feast and added to the tension that would lead to Ragnarök, an event Heimdall is closely associated with as the watchman of the gods. The correct option is D.
Step-by-step explanation:
The god that showed up last at Frey's feast and threatened to kill Loki for his betrayal is none other than Heimdall. This event is depicted in Norse mythology, specifically in the tale of Ægir's feast which is described in the Poetic Edda, a collection of Old Norse poems from the medieval manuscript Codex Regius. According to the myth, Loki was already not welcome at the feast due to his previous mischief and betrayals. When he forces his way into the hall, he insults all the gods present, including Odin, Freyja, and Thor. Eventually, Heimdall stands up to Loki and threatens him, which contributes to the increasing tension at the feast and preludes to the chain of events leading to Ragnarök, the end of the world in Norse lore.
Heimdall is known as the watchman of the gods and is destined to sound the Gjallarhorn, signaling the beginning of the end during Ragnarök. His role in the mythology is pivotal as he keeps watch for the giants and is responsible for guarding Bifröst, the rainbow bridge connecting Midgard (the world of humans) and Asgard (the realm of the gods).