Answer:
C is correct.
Explanation:
E has idiomatic issue: "decline at" is wrong. Also another major issue is lack of parallelism:
Japan’s population is shrinking faster than that of any other nation, with a projected decline at
Portion underlined is not parallel. First is a possessive noun, and second is not. So although the comparison is not an issue, parallelism is not maintained. So E is wrong.