B is correct ....

It's a translation, but that's as close as it gets: River bank or quay ahead ...

I only can give you the advice to make exercices on the site. Ther are more than 1000 questions, up to date with the real exam.

But then why does the book say C is correct?
https://imgur.com/a/IOOzURq

Answer B is correct.

I thought this is the website that the book is from?
But can someone advise me?
Is that my mistake or the book's?

The book with exercices is not from the site.
There are more than 1000 questions on the site.

I'm working through the English "Theory Driving Licence B" book, in the Lesson 1 practice questions. One question asks which sign indicates a river bank ahead, showing sign A (movable bridge) and sign B (car going off an edge into water), with the marked answer being C – neither sign A nor sign B.
The car‑into‑water sign is standardly described in English as "quayside *or* riverbank" (your own online materials also list it as "warning for a quayside or riverbank").

so why is the correct answer C?

https://imgur.com/a/k721Ka1