The project manager must take immediate action to address both behavioral and performance issues.
Providing direct, constructive feedback to the underperforming senior technical member will help address their critical behavior and performance lapses. According to the PMBOK Guide - Seventh Edition, project managers are responsible for managing interpersonal conflicts and promoting team collaboration.
By addressing the disruptive behavior and lack of task performance, the project manager fosters a psychologically safe environment and restores team dynamics.
Option A avoids the issue and reduces collaboration.
Option B escalates prematurely and may cause greater disruption.
Option C is positive, but without addressing the root cause, the issue may persist.
References:
PMBOK Guide - Seventh Edition, Team Performance Domain
PMI Talent Triangle - Leadership: Conflict and Feedback Management
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