According to the syllabus, root cause analysis is a technique that aims to identify and address the underlying causes of defects or problems in a process or system. Root cause analysis involves finding the effect (the observable symptom or outcome of the problem), finding the cause (the factor or factors that directly influence the effect), and finding the root cause (the fundamental reason why the cause exists). The answer C is correct because it most accurately reflects this scenario. The incorrect customer discount is the effect and the reason for the requirement error is the root cause. The other answers are incorrect because they either confuse the effect, cause, and root cause or use incorrect terms.

References: [Certified Tester Foundation Level Syllabus], Section 5.2.1, page 58.