A project manager has instructed the quality team to focus on the top 20 percent of causes leading to the identified defect so as to prevent the problem from happening again. What does this action exemplify?

The 80/20 rule, a.k.a Pareto’s Law or the Pareto Principle, states that: "80 percent of problems result from 20 percent of causes". This is exactly what the project manager in this scenario is performing.
Specification limits and assignable cause are related to control charts.
Trend analysis relates to run chart.