The book represents a wonderful love story so once it will be popular with young people.

问题补充：

The book represents a wonderful love story, so once, it will be popular with young people.

A. publishes

B. publishing

C. published

D. having published

答案：

A. publishes

B. publishing

(答案→)C. published

D. having published

解析：once作为连接词引导条件状语从句, 后面可以接现在分词或过去分词,由于it与publish为被动关系, 所以用过去分词published作状语。