The best definition of the word rude as it is used in the first stanza is:
C) rough, unfinished
Step-by-step explanation:
The motif used here to convey the imagery of the unfinished or the treacherous bridge here is done by the connotative meaning of the word rude and the emotion it conveys.
The word rude is used to show the treacherous nature of the bridge in that it could fall to the storm and is harmful for anyone that is near it or above it as the flag is right there on the bridge unfurled.
This is the best meaning of the word 'rude' and how it works in the poem.