Final answer:
Thor is likely to have left Megingjord, his belt of strength, in Odin's palace Valaksjalf when he visited Gerriod. Among the items listed, Megingjord is the most plausible since it was an accessory used to amplify Thor's power during challenging encounters. Option 3: Megingjord is the correct answer.
Step-by-step explanation:
In Norse mythology, Thor, the god of thunder, is known for leaving Mjolnir, his powerful hammer, in Odin's palace Valaksjalf when he went to the dwelling of the giant Gerriod. However, he also had other significant items associated with his persona, including the belt of strength Megingjord, his iron gloves, and the less commonly mentioned weapon, Jarnbjorn. When Thor visited Gerriod's abode, he did so without some of his most powerful equipment to show humility and to catch Gerriod off guard.
Among the options provided, Option 3: Megingjord, Thor's belt of strength that amplified his power when worn, is the most likely item that was left behind. This is because Thor often wore Megingjord when going into battle or facing formidable foes, and it was an essential accessory that augmented his already considerable might. The other items listed, such as Mjolnir and Gungnir (which is actually Odin's spear), were either essential to Thor's battles or not directly associated with him.