What does vertical traceability trace?

Traceability links requirements to other project artefacts. There are two main types: horizontal and vertical. Horizontal traceability (Option A) links a requirement forward to its design, code, and test cases, and backward to its source (Option D), tracing its lifecycle from inception to delivery. Vertical traceability concerns tracing a requirement up or down the requirements hierarchy (C). This ensures that lower-level requirements (e.g., functional) are aligned with and fully support the higher-level business objectives, policies, and strategy. For example, it traces a solution requirement up to the business requirement it satisfies, validating that every low-level detail is necessary and justified by a high-level business need.

(Reference: BCS Requirements Engineering – Requirements Traceability)