It is best to coach the Development Team members to collaborate on issues regardless of severity. They may consult the Scrum Master of Product Owner if needed.

Multiple Scrum Teams could have the same Sprint start dates but it is not mandatory as the importance is on being able to integrate their shippable Increments at the end of their Sprints.

Velocity only measures the team's average speed and does not reflect how much value is being delivered to the customers. Even when it's considered inside the project, while it's important, it's not the key measure of success. The Product Owner should be focused on how much value is delivered to the customers.

Scrum is founded on empirical process control theory, or empiricism. Empiricism asserts that knowledge comes from experience and making decisions based on what is known. The Sprint Backlog is a forecast by the Development Team about what functionality will be in the next Increment and the work needed to deliver that functionality into a 'Done' Increment. The Development Team modifies the Sprint Backlog throughout the Sprint, and the Sprint Backlog emerges during the Sprint. This emergence occurs as the Development Team works through the plan and learns more about the work needed to achieve the Sprint Goal.
Why C is incorrect:
a) The Dev Team does not commit to finishing all items in the Sprint Backlog. Committing to completing all items would be fixed scope and fixed time leaving no room to adapt. They commit to the Sprint Goal and doing the right thing.
b) It's also incorrect for the PO to focus on completing the entire Sprint Backlog. Finishing everything should not be his main focus. Having a shippable increment that meets the Sprint Goal is more important than trying to finish everything. The advice in option C is equivalent to "Don't worry, they'll get everything done." but because of the complex domain, you cannot guarantee everything will get done.

Without trust, team members will have difficulties acting and behaving in the ways that reflect the Scrum values and how effective they apply empiricism.