An Approval Process is the dedicated Salesforce feature for managing workflows that require human "review and decision-making." Unlike Assignment Rules (Option A), which simply change the owner of a record, an Approval Process locks the record to prevent further changes and routes a formal request to an "Approver." This approver can then choose to Approve, Reject, or Reassign the request. The process can include multiple steps, entry criteria (e.g., "only if discount > 10%"), and specific automated actions that occur once the final decision is made.
Validation Rules (Option B) are used to prevent saving bad data, not for routing. Schema Builder (Option D) is a visualization tool for the data model. Therefore, when a business process requires an official "sign-off" or "decision," the Approval Process engine is the correct architectural choice.