You're a Workspace Admin on a Slack Enterprise Grid plan.

Slack best practices recommend:

"For external users who need access to multiple channels but not the entire workspace, a Multi-Channel Guest account is the ideal solution."

Full member access (D) is inappropriate for external users. Slack Connect DMs (B) would not provide full channel collaboration. Single-Channel Guest (C) would limit the contractor unnecessarily.

(Reference: Slack Administration Study Guide - Managing Guest Accounts in Enterprise Grid)

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