Explanation
According to the House of Lean article on the Scaled Agile Framework website, the principle that explains the purpose of the PI Planning retrospective is relentless improvement. The article states that "Relentless improvement is a constant sense of danger combined with a paranoid desire to find a better way. It's a culture of organizational self-assessment, problem-solving, and action. It's a willingness to change before the crisis forces us to do so. It's a commitment to relentless reflection and a continuous learning journey." Therefore, the correct answer is D, relentless improvement. The other options are not accurate, as they are not the principles that directly relate to the PI Planning retrospective. Respect for people and culture is the foundation of the House of Lean, innovation is the roof, and flow is another principle that supports value delivery.