Part A: The most likely conclusion about Beowulf that can be drawn from his battle with Grendel's mother is: D. Beowulf triumphed over Grendel's mother through a combination of good fortune and his own strength and determination.
This conclusion reflects the fact that Beowulf eventually finds a powerful sword that he is able to wield, which suggests an element of luck in discovering that weapon, in addition to his determination and physical strength.
Part B: The excerpt from Beowulf that best supports the answer to Part A is: D. Then he saw, hanging on the wall, a heavy / Sword, hammered by giants, strong / And blessed with their magic, the best of all weapons / But so massive that no ordinary man could lift it. / [He] lifted it high over his head / And struck with all the strength he had left. (lines 528-536)
This passage clearly indicates that Beowulf's triumph involved discovering the magical sword, showcasing both his fortunate circumstances and the strength he had to wield it in his final confrontation with Grendel's mother.