A have a sentence in a test book: This must be the worst travel book.....has ever been published.

The right answer(in the book)is:"that"
why? is this the only acceptable answer?"Which "is not good?

Anna,
this is an excellent question. The answer is:
"that" is okay
"which" is also okay (note: no comma before "which" in this case.)

the clause "that/which has ever been published" is required to identify the book. Therefore, it is not preceded by a comma. This topic is covered here.

In short:

that = which (without a comma)
that does not equal ,which

I always have a problem when teaching conjunctions. They are varied in many ways. Ss find them difficult to use and memorize.