Who is responsible for ensuring the Product Backlog items are understood to the level needed?

Steven, the Scrum Master, is approached by one of the Development Team members saying that they are not completing regression tests for all of the work they are performing to the level defined in the Definition of Done.
They have discussed this with the Product Owner and decided to remove regression testing from the Definition of Done.
Which two actions are the most appropriate for Steven to take? (Choose two.)

Who manages the progress of work during a Sprint?

At the Sprint Planning, the Development Team is not able to forecast the number of Product Backlog items it can do in the upcoming Sprint due to unclear requirements. The Product Owner, however, was able to clearly define the business objective he hopes to achieve in the Sprint.
Which of the following two actions would you support? (Choose two.)

A Development Team member has notified the Scrum Master of a potential data security risk.
What action should the Scrum Master take?