The correct answer is A, as it states that the Wideband Delphi estimation technique is an example of the expert-based approach. The Wideband Delphi estimation technique is a method of estimating testing effort or duration by using a structured group process that involves multiple experts2. The experts provide their estimates independently and anonymously, then compare and discuss them until they reach a consensus2. This technique is an example of the expert-based approach, which is an approach that relies on the knowledge and experience of experts to estimate testing activities2. Option B is incorrect, as it states that the Wideband Delphi estimation technique is an example of the metrics-based approach. The metrics-based approach is an approach that uses historical data and mathematical formulas to estimate testing activities2. This approach does not involve experts or group processes. Option C is incorrect, as it states that burndown charts used in Agile development is an example of the expert-based approach. Burndown charts are graphical tools that show the amount of work remaining versus time in an Agile project2. They are used to monitor and control testing progress and quality. They are not examples of the expert-based approach, as they do not rely on experts' opinions or estimates. Option D is incorrect, as it states that burndown charts used in Agile development is an example of the risk-based approach. The risk-based approach is an approach that uses risk analysis to prioritize and estimate testing activities2. This approach involves identifying and assessing risks based on their likelihood and impact. It does not involve burndown charts or Agile development. References: 2, Section 2.6