Final answer:
To make Liam's narrative more precise, the sentence 'I had finally managed to dive from the highest platform' best replaces sentence 5 by clearly stating the specific accomplishment of diving from the highest platform.
Step-by-step explanation:
The student is asking for a more precise sentence to replace sentence 5 in Liam's narrative to better convey the outcome of his experience. The best choice among the given options is "I had finally managed to dive from the highest platform." This sentence is more specific than the original sentence 5 as it details the exact accomplishment Liam achieved, which is diving from the highest platform. The other options are either too vague or introduce a new idea without the precision required to specify the achievement detailed earlier in the paragraph.