(a) Error: More / Correction: Most
"Most of the fun…" is correct as it refers to the greatest part (superlative), not a comparison between two things.
"More" is a comparative degree used when comparing two things. Here, the sentence refers to the greatest portion of fun and excitement in life generally, so the superlative "Most" is required. Examiners expect you to identify the exact word and give the correct replacement clearly.