This scenario highlights a common project risk: resource dependency on a single skilled team member. The most immediate and practical action is to ensure active collaboration among team members to foster knowledge transfer. Agile and adaptive environments encourage continuous collaboration to mitigate such risks early. Option D reflects proactive risk response and aligns with the Agile principle of shared knowledge and team empowerment.

The Agile Practice Guide (Chapter 4: "Agile Teams and Team Performance") emphasizes the importance of collaboration and cross-functional learning. It promotes the development of T-shaped skills, where individuals develop a broad range of knowledge while maintaining deep expertise in specific areas. This approach allows other team members to step in when needed.

Option A is too long-term and not responsive enough to the current project phase.

Option B might not address the knowledge gap if others are untrained.

Option C is useful but does not solve the immediate knowledge-sharing concern.