A lesson learned session should be held at what point in a project?

A.
At the project conclusion

B.
At key milestones

C.
When risk event occur

D.
When sponsor want

Explanation:
Milestone is an event that receives special attention. It is often falsely put at the end of a stage to mark the completion of a work package or phase. But milestones are rather to be put before the end of a phase so that corrective actions in case of problems can still be met and the deliverable can be completed in time.
In addition to signaling the completion of a key deliverable, a milestone may also signify an important decision or the derivation of a critical piece of information, which outlines or affects the future of a project. In this sense, a milestone not only signifies distance traveled (key stages in a project) but also indicates direction of travel since key decisions made at milestones may alter the route through the project plan.