Final answer:
The most likely symbolism of the contrast between the sword Beowulf used to kill Grendel's mother and how it melted after beheading Grendel with it is the idea of both triumph and vulnerability.
Step-by-step explanation:
The most likely symbolism of the contrast between the sword Beowulf used to kill Grendel's mother and how it melted after beheading Grendel with it is the idea of both triumph and vulnerability. The sword represents Beowulf's strength and ability to triumph over evil, as demonstrated by his success in killing Grendel and his mother. However, the fact that the sword melts and is described as blood-soaked and dripping down like water highlights the temporary nature of victory and the ephemeral nature of power and heroism.