Towards the end of Sprint Planning, the Product Owner and the Developers have not been...

A. They discuss in the upcoming Sprint Retrospective why this happened and what changes will make it less likely to recur. This is a good action to take, as it shows that the Scrum Team is willing to inspect and adapt their process and improve their collaboration and communication. The Sprint Retrospective is an opportunity for the Scrum Team to identify the root causes of the problem and create a plan for implementing improvements in the next Sprint.

D. The Developers forecast the most likely Product Backlog items to meet the business objective and create a Sprint Backlog based on a likely initial design and plan. Once the timebox for the Sprint Planning event is over, they start the Sprint and continue to analyze, decompose, and create additional functionality during the Sprint. This is also a good action to take, as it shows that the Scrum Team is flexible and adaptive, and does not let uncertainty or ambiguity prevent them from delivering value. The Developers can use the Sprint Goal as a guide to select and refine the Product Backlog items during the Sprint, and collaborate with the Product Owner to ensure that they are meeting the expectations and needs of the stakeholders.

References:

The Scrum Guide, section 3.1 (The Sprint), page 8

The Scrum Guide, section 3.3 (The Sprint Goal), page 9

The Scrum Guide, section 3.5 (The Sprint Planning), page 10

The Scrum Guide, section 3.7 (The Sprint Retrospective), page 12

The Scrum Master Learning Path, module 2 (The Scrum Framework), lesson 2 (The Sprint), lesson 3 (The Sprint Goal), lesson 4 (Sprint Planning) and lesson 6 (The Sprint Retrospective)