Answer:
The last obstacle in the protagonist's journey/story.
Explanation:
Since there isn't much context to the story, I am assuming that "The last rung on the ladder" could either have a literal or figurative meaning. If it's a literal meaning, the character might actually be climbing a ladder and they're about to take the last step (the rung) to get to the top. However, it's more probable that the author means it in a figurative sense. I believe that "The last rung" is referring to a series of conflict in the protagonist's life, and the author refers to the ladder and its rungs as the conflicts.
Protagonist's story - the ladder (as a whole)
Conflicts faced - the rungs of the ladder (hence "The last rung on the ladder"
* Hopefully that made sense :))
