According to the PMBOK® Guide, the process described is Estimate Costs. This is the process of developing an approximation of the monetary resources needed to complete project work.

Purpose: The key benefit of this process is that it determines the monetary resources required for the project. These estimates are expressed in units of currency (e.g., dollars, euros, etc.) to facilitate comparison between activities and projects.

Accuracy over Time: Cost estimates are refined throughout the project. For example, a project in the initiation phase may have a Rough Order of Magnitude (ROM) estimate in the range of −25% to +75%. Later in the project, as more information is known, estimates could narrow to a Definitive Estimate range of −5% to +10%.

Inputs and Tools: This process uses inputs such as the project management plan, project documents (like the lessons learned register and project schedule), and enterprise environmental factors. Common tools include Analogous, Parametric, Bottom-up, and Three-point estimating.

Why other options are incorrect:

Option A: Plan Cost Management: This is the process that establishes the policies, procedures, and documentation for planning, managing, expending, and controlling project costs. It defines how costs will be estimated, not the actual estimates themselves.

Option B: Estimate Activity Resources: This process (part of Project Resource Management) is about identifying the types and quantities of material, human resources, equipment, or supplies required. While it is a precursor to estimating costs, it focuses on the physical/human requirements rather than the monetary approximation.

Option D: Determine Budget: This is the process of aggregating the estimated costs of individual activities or work packages to establish an authorized cost baseline. Estimating the individual resource costs (Option C) must happen before they can be aggregated into a budget.