Answer:
A)He tells stories of his previous battles with men and monsters.
Step-by-step explanation:
The proof that Beowulf offers to show that he is up to the challenge of killing Grendel is to tell stories of his past victories over mighty men and other monsters.
Beowulf is a mighty warrior who comes to the aid of a country that is tormented by the monster Grendel so he talks about his achievements to prove that he is capable of killing Grendel.