Final answer:
Iron blades are traditionally unusable to Beowulf due to his exceptional strength causing them to break upon use, not for the reasons listed in the multiple-choice options.
Step-by-step explanation:
According to legend, iron blades are useless to Beowulf not because of any of the options provided in the multiple-choice question, but because Beowulf's own strength is considered greater than any weapon's might. The legendary hero is fabled to have an immense natural strength, so much so that the weapons made of iron would break upon use due to the force of his combat. In the battle against Grendel, a fearsome monster from the epic, Beowulf chooses to fight without a weapon, further emphasizing his inherent power. The text provided does not directly answer the multiple-choice question, but it suggests that Ethelred faces the dragon, and the mention of a brazen shield suggests that metals do indeed have significance in the story. However, as deducted from the larger narrative of Beowulf, one could figure out iron's impotence in aiding him during battle is due to his extraordinary strength.