According to the Scrum Guide, the action that the Scrum Master should take when the Team decides that a Sprint Retrospective is unnecessary is to begin facilitating productive and useful Sprint Retrospectives. The Sprint Retrospective is an essential event for the Scrum Team to inspect itself and create a plan for improvements to be enacted during the next Sprint. The Scrum Master is responsible for ensuring that this event takes place and that positive outcomes are achieved. The other options are not valid actions, as they either imply that the Scrum Master complies with or ignores the decision of the Team (such as complying with it, calling a meeting with senior management, or consulting with the Product Owner), which is not consistent with Scrum values and principles.

The correct answer is B, because there is no role called "Project Manager" in Scrum. The Scrum Guide states that "Scrum recognizes no titles for Development Team members other than Developer, regardless of the work being performed by the person; there are no exceptions to this rule." Therefore, Scrum has only three roles: Product Owner, Scrum Master, and Developer.

According to the Scrum Guide, the Scrum Team is responsible for removing a Developer from the Scrum Team if necessary, as they are self-managing and accountable for creating valuable Increments. The other options are not valid, as they imply that someone else outside the Scrum Team has the authority to remove a Developer (such as the Scrum Master, the Product Owner, or the hiring manager), which is not consistent with Scrum values and principles.