Final answer:
In the story, Grendel cannot approach Hrothgar on his throne because it is protected by a dragon.
Step-by-step explanation:
The throne of Hrothgar was protected by a dragon in the story. In the passage, Grendel is unable to approach Hrothgar on his throne because a dragon of a scaly and prodigious demeanor was guarding it. The dragon sat in guard before a palace of gold, protected by its fiery tongue and imposing presence.