acceptance criteria

I can't find the description of acceptance criteria in scrum guild
but it seems that when I take exams, I see them

Since they are not referred to in the Scrum Guide, that tells you they will not be essential to a correct answer.

Which "exams" were referring to them?

As Ian mentioned, the Scrum Guide says nothing about how PBI's should be captured or articulated, so no official Scrum exam should be expecting you to talk about this. 



That said, it's important that the Product backlog is transparent and regularly adapted, and so *if* the team are using acceptance criteria as a way to capture the details of the customer needs, it would be a pretty good idea to make sure this is done before the team is expected to start work. 



Refinement should be an ongoing activity, and so is the natural place for these conversations and backlog updates to occur. 



As for *who*, think about the term *acceptance*. Who accepts the work? Hopefully this is our customers and stakeholders.  



This means accountability will ultimately be with the Procut Owner, who is accountable for maximizing the value of the work, and who represents the customer when we don't have one available. That doesn't mean they cannot delegate the actual creation of acceptance criteria, however. 

As others have already mentioned, there is no mention of acceptance criteria in the Scrum Guide.

I am wondering if you are preparing for the PSD (Professional Scrum Developer) exam. If so, you will need to look outside of the guide and study complimentary development practices in addition to the guide.

I mention this as there may be questions on Acceptance Test-Driven Development (ATDD), as an example, which does leverage and reference acceptance criteria. 

From the Professional Scrum Developer Glossary:

Acceptance Test-Driven Development (ATDD): test-first software development practice in which acceptance criteria for new functionality are created as automated tests. The failing tests are constructed to pass as development proceeds and acceptance criteria are met.

If you are preparing for another exam, it would be helpful for you to share which one and what practice exams you are leveraging in order to better support your question.