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.