Which of the following is an attribute considered within Requirements Viewpoints?

Model notations are an essential attribute within Requirements Viewpoints because they describe the visual representation and formalism used to articulate the requirements. This aspect of requirements analysis ensures that stakeholders interpret the requirements in a consistent and structured manner. Model notations facilitate communication among various stakeholders by providing a common language or symbols to represent complex ideas, processes, or systems graphically.

The other options, while related to the broader context of project management and requirements gathering, do not specifically fall under the category of attributes considered within Requirements Viewpoints. Timeline projections relate to the scheduling of project phases, stakeholder interviews focus on gathering input from people involved in the project, and budget estimates deal with financial aspects—none of which pertain directly to the structural representation of requirements themselves as model notations do.