Product owner role - 2 activities part of the product owner role

Hello, 

I have a debate about the product owner's role. An open assessment question says :

"ensuring that most valuable functionalities are produced first, at all time."

Although, I understand it is the product owner's role to set the priorities in the product backlog, it seems to me that it is down to the development team to pull the item, and to choose it for being part of a sprint.