Which of the following best describes Cost Management?

Cost Management encompasses a range of processes and activities that are essential for ensuring a project is completed within its approved budget. This includes both cost estimation, which involves forecasting costs associated with all project activities, and project budget, which entails the allocation of those estimated costs to particular project tasks in order to keep track of expenditures and overall financial performance. The approach central to Cost Management aims at effectively planning and controlling project costs throughout its lifecycle, rather than solely focusing on a single aspect or taking a narrow view.

The other options don't encompass the full scope of Cost Management. Focusing exclusively on cost-cutting measures overlooks crucial elements such as accurate budgeting and cost estimation. Similarly, reviewing project designs extensively is an important part of project planning, but it pertains more to project management aspects outside of direct financial considerations. Lastly, while managing financial matters with external vendors is certainly a component of a project's financial oversight, it is just one piece of the broader framework of managing all costs associated with a project. Overall, understanding both estimation and budgeting forms the backbone of effective Cost Management, making it a vital part of project success.