According to the Scrum Guide1, the Developers are self-organizing and decide how to turn Product Backlog items into an Increment of value. No one tells them how to do their work. The Developers are accountable for creating a plan for the Sprint, the Sprint Backlog. The other roles are not responsible for making sure everyone on the Development Team does their tasks for the Sprint.

The correct answer is D, because one of the services that a Scrum Master can provide in regard to the Daily Scrum is to teach the Development Team to keep the Daily Scrum within the 15 minute time-box. The Scrum Guide states that "the Scrum Master ensures that the Development Team has the meeting, but the Development Team is responsible for conducting the Daily Scrum. The Scrum Master teaches the Development Team to keep the Daily Scrum within the 15-minute time-box."

According to the Scrum Guide1, the definition of "Done" is a formal description of the state of the Increment when it meets the quality measures required for the product. The definition of "Done" is created by the development organization (or Development Team if none is available from the development organization). The definition guides the Development Team in creating a "Done" Increment.

The correct answer is B, because when multiple Scrum Teams work from the same Product Backlog, they should pull in work in agreement with the Product Owner. The Scrum Guide states that "multiple Scrum Teams often work together on the same product. One product goal unites them. Therefore, they must mutually define and comply with a definition of 'Done' that applies to their combined work product." Therefore, they should collaborate and coordinate with each other and with the Product Owner to select Product Backlog items that align with their shared product goal.

According to the Scrum Guide, the Definition of Done is a formal description of the state of the Increment when it meets the quality measures required for the product1. The Definition of Done is created by the Developers and it is used to assess when work is complete on the product Increment1.
The Scrum Guide also states that the Definition of Done may vary significantly per Scrum Team, depending on the context. The Definition of Done evolves over time and becomes more stringent as the Scrum Team matures1. Therefore, it is important for the Scrum Team to regularly inspect and adapt the Definition of Done to reflect their current situation and needs.
The Sprint Retrospective is an opportunity for the Scrum Team to inspect itself and create a plan for improvements to be enacted during the next Sprint1. The Sprint Retrospective focuses on three questions: What went well in the Sprint? What could be improved? What will we commit to improve in the next Sprint?1 These questions can help the Scrum Team identify potential changes to the Definition of Done that would increase the quality and value of the product.
Option A is incorrect because Product Backlog refinement is not a formal Scrum event, but rather an ongoing activity that can happen at any time during the Sprint1. Product Backlog refinement is mainly concerned with adding detail, estimates, and order to Product Backlog items1. Changing the Definition of Done during Product Backlog refinement could cause confusion and inconsistency among the Developers and the Product Owner.
Option C is incorrect because Sprint Planning is a time-boxed event that initiates a Sprint by defining a Sprint Goal and selecting Product Backlog items that deliver one coherent function1. Sprint Planning is based on the current state of the Product Backlog and the latest Definition of Done1. Changing the Definition of Done during Sprint Planning could disrupt the flow of the event and affect the scope and feasibility of the Sprint.
Option D is incorrect because prior to starting a new project is not a specific time in Scrum. Scrum does not prescribe any phases or stages for a project, but rather an empirical approach that relies on frequent inspection and adaptation of both the product and the process1. Changing the Definition of Done before starting a new project could be premature and irrelevant, as the Scrum Team may not have enough information or experience to define what 'Done' means for their product.