Final answer:
The valid reason for moving a PI objective to stretch is because the objective is too challenging. Stretch objectives serve as aspirational goals without impacting the original commitments of the Agile team.
Step-by-step explanation:
The question pertains to moving PI (Program Increment) objectives to stretch in an Agile project management framework. A valid reason for moving an objective to stretch is c) The objective is too challenging. Stretch objectives are used when a particular goal is desired but recognized as being beyond what the team can commit to with high certainty. They serve as an aspirational target that the team can strive towards if they meet their committed objectives ahead of schedule. It is not usual to move an objective to stretch simply because more time is needed or because stakeholders request it, nor is it done when an objective is easily achievable as that would defeat its purpose. The idea is to motivate and provide a direction for continuous improvement without affecting the predictability and commitments of the team.