Who is responsible for clearly expressing Product Backlog items?

According to the Scrum Guide1, the Product Backlog is an ordered list of everything that is known to be needed in the product. It is the single source of requirements for any changes to be made to the product. The Product Owner is accountable for maximizing value resulting from work by Developers. The Product Owner is also accountable for effective Product Backlog management, which includes:

Developing and explicitly communicating the Product Goal;

Creating and clearly communicating Product Backlog items;

Ordering Product Backlog items; and

Ensuring that the Product Backlog is transparent, visible and understood.

Therefore, the Product Owner is responsible for clearly expressing Product Backlog items.

References: Scrum Guide