Correct answer: A. Root cause analysis of defects
Explanation: The direct answer is A: root-cause analysis of defects will be difficult. A final-product inspection tells the factory whether the finished item is acceptable, but it gives limited information about when, where, and why the defect entered the process. Root-cause analysis requires evidence from earlier stages, such as machine settings, material quality, worker actions, and process conditions. If those stages are not observed or recorded, investigators must guess or rely on incomplete evidence. Option A is correct because the missing process information directly blocks finding the underlying cause. Option B, office colour, is unrelated to manufacturing defects. Option C, employee birthdays, has no technical connection with product quality. Option D, customer income, may matter in marketing analysis but cannot explain where a production defect began. A stronger method combines final inspection with checks during important process stages. Memory cue: finding the source requires observing the process, not only the result.