To enable Microsoft Teams shared channels for collaboration with an external organization, configuration is required at both the identity platform level (Microsoft Entra ID) and the application level (Microsoft Teams).

First, a B2B direct connect cross-tenant access policy must be configured in the Microsoft Entra admin center. This establishes a mutual trust relationship, and specifically, the outbound access settings must be configured to allow your users (the marketing department) to be added to the external organization's resources.

Second, a Teams policy must be assigned to the marketing department users. This policy must explicitly grant them the permission to invite external users to the shared channels they create and manage, enabling the desired collaboration.

B. This option is identical to option C. In a valid exam, duplicate options would not appear. Both describe the necessary Microsoft Entra configuration.

D. While allowing users to join external shared channels is a valid policy, the primary action to initiate collaboration from your organization's side is enabling the invitation of external users.

E. The requirement is specific to the marketing department. Applying a policy to the entire organization via the "Global (Org-wide default)" policy is incorrect and overly permissive.

1. Microsoft Learn. Collaborate with external participants in a channel. Microsoft Docs. "To set up shared channels with another organization, you need to... 1. Enable shared channels in Teams... 2. Configure cross-tenant access settings for B2B direct connect." This document explicitly states the two high-level steps that correspond to answers A and C.

2. Microsoft Learn. B2B direct connect in Microsoft Entra ID. Microsoft Docs. "To allow your users to collaborate with people in another organization using B2B direct connect, you must... 1. Add an organization... 2. Configure outbound settings for the organization to allow your users to access their resources." This reference details the action described in option C.

3. Microsoft Learn. Manage channel policies in Microsoft Teams. Microsoft Docs. This document lists the specific Teams policies, including "Invite external users to shared channels," and explains that these can be applied to specific users or groups, supporting the targeted approach in option A over the global approach in option E.