The sentence that most likely serves as the climax of a story is option 2 - I was able to grab the rope just as the canoe full of kittens was slipping away from the dock.
Of the four options provided, this sentence contains the most tension. It suggests there was a chase to save the kittens before they floated out of reach. The other sentences don't necessarily mark pivotal moments in a story or convey any form of excitement, nervousness, or anticipation.
