Your organization is choosing an identity provider (IdP) and preparing to launch Slack Enterprise Grid.

Slack requires:

"Enterprise Grid must use an identity provider (IdP) to manage authentication via SSO before users can sign in. SSO must be configured first for member access."

Options A and B bypass the necessary authentication process and would create user confusion later. Downgrading to Business+ (C) defeats the purpose of deploying Enterprise Grid.

(Reference: Slack Administration Study Guide - Preparing Identity Management for Enterprise Grid)