Final answer:
The Developers are accountable for estimating the effort to complete the Product Backlog items.
Step-by-step explanation:
The accountable party for estimating the effort to complete the Product Backlog items is The Developers.
Although the Product Owner manages the Product Backlog, it is the Developers who are responsible for providing input on the effort required to complete the items in the backlog.
They leverage their technical expertise and understanding of the project to estimate the time and resources needed for each item.