Final answer:
The sentence is a simile comparing an issue to a painful sore because it uses the word 'like' to draw the comparison, which is characteristic of similes.
Step-by-step explanation:
The sentence "Like a boil that can never be cured as long as it is covered up but must be opened with all its pus-flowing ugliness" is most accurately described as b) A simile comparing an issue to a painful sore. This sentence uses the word 'like' to draw a comparison between the approach to dealing with a boil and confronting issues, typical of how similes function to compare two different ideas or things. Given the provided context of similes in literature and the provided information about boils and carbuncles which are medical conditions needing treatment, this comparison lends itself to the figurative language of a simile.