A new contractor joining the project team is an example of:

Offshoring specifically means relocating work or business processes to another country, which is not implied by simply adding a contractor to a team.

Offboarding is the process of transitioning a team member out of the project or organization, the opposite of onboarding a new contractor.

Rightsizing refers to optimizing the organizational or team structure to an appropriate scale for efficiency, not the act of engaging an external contractor.

Outsourcing is the practice of contracting work to an external individual or organization rather than performing it with internal staff. A contractor joining the project team is precisely this arrangement - the organization is leveraging an outside resource to fulfill a project role, which is the textbook definition of outsourcing in project management.