The best anser is C - gaunt. Gaunt means thin and looking exhausted, such as when one is old or tired.
The answer can be actually found directly in the poem, which contains the line: 'An aged thrush, frail, gaunt, and small'. Gaunt is used directly in the poem!
What story is it and I might be able to help
The swallow's body most likely symbolizes Love and devotion<span />