According to the PMBOK® Guide and the Agile Practice Guide, the nature of stakeholder engagement changes significantly when moving from a predictive (waterfall) to an adaptive (agile) lifecycle.

Continuous Involvement: In agile projects, key stakeholders (including customers and product owners) are continuously involved. They do not just provide requirements at the beginning and check the results at the end; they provide ongoing feedback, clarify requirements, and participate in iterative reviews.

Frequency of Interaction: High-frequency interaction reduces the risk of building the wrong product. By being continuously involved, stakeholders can see the product as it grows, allowing them to request changes or pivot the project ' s direction based on real-time learning.

Collaborative Environment: Adaptive environments emphasize " Customer Collaboration over Contract Negotiation. " This requires a partnership where stakeholders are integrated into the rhythm of the project, often participating in Daily Stand-ups, Sprint Reviews, and Backlog Refinement.

Why other options are incorrect:

Option A: Key stakeholders are regularly involved: While " regularly " implies a pattern, it doesn ' t quite capture the " always-on " nature of agile. In agile, the involvement is tighter than just " regular " intervals—it is a continuous loop.

Option C: Key stakeholders are involved at specific milestones: This is a characteristic of Predictive (Waterfall) lifecycles. In those projects, stakeholders are often only engaged during major phase gates or milestone approvals, which can lead to significant gaps between expectations and reality.

Option D: Key stakeholders are always involved: While it sounds similar to continuous, " always " can be misleading in a professional context. Stakeholders are not literally present 24/7 (as " always " might imply), but their feedback and presence are continuous throughout the iterative process. " Continuously " is the formal term used by PMI to describe the active, ongoing engagement model.