What artifact from the original project should be leveraged to determine the purpose of the class?

A year has passed since a project has gone live and a developer is looking to make an update to an existing Apex class, but is unsure of its purpose.

What artifact from the original project should be leveraged to determine the purpose of the class?
A . User Acceptance Test Scripts
B. Test Execution Plan
C. Requirements Traceability Matrix
D. Test Sign Off Document

Answer: C