Which area of project management covers the blending of various subteams into a project organization with a cohesive plan?

Integration management, as defined in the PMBOK Guide, is concerned with identifying, combining, unifying, and coordinating the various processes and activities across all project knowledge areas and subteams. It ensures all elements of the project - including separate subteams - operate under one cohesive plan. This directly matches the question's description of blending subteams into a unified project organization.

Human resources management focuses on organizing, managing, and leading the project team members - not on blending separate subteams and their individual plans into a unified project structure.

Risk management is concerned with identifying, analyzing, and responding to project uncertainties and threats, not with integrating subteams or harmonizing plans.

Communications management addresses the planning and distribution of project information to stakeholders, not the organizational blending of subteams into a cohesive plan.