Microsoft Exchange Online supports converting an existing user mailbox to a shared mailbox while retaining its historical email content. This is appropriate when the original user no longer needs direct access but other employees must continue using the mailbox.

Option C is correct because converting Mailbox1 into a shared mailbox preserves the existing email messages and enables delegated access for other users.

The mailbox contains 42 GB of data, which is below the standard 50 GB unlicensed shared mailbox limit. Therefore, an additional mailbox license is not required solely because of its current size, provided no separately licensed features are necessary.

Option B is correct because Full Access permission allows Group1 members to open the mailbox and manage existing items. Send As permission enables them to send messages that appear to originate directly from Mailbox1.

Option E is incorrect because Send on Behalf permission displays the individual delegate ' s identity, whereas the requirement specifies that messages must appear to come from Mailbox1.

Option D is incorrect because Litigation Hold is designed for preservation and compliance purposes, not for providing collaborative mailbox access.

Option A is unnecessary for a standard shared mailbox of this size.

Therefore, converting the mailbox and granting Full Access and Send As permissions satisfies all three requirements.

Microsoft References:

https://learn.microsoft.com/microsoft-365/admin/email/convert-user-mailbox-to-shared-mailbox

https://learn.microsoft.com/en-us/exchange/collaboration/shared-mailboxes/create-shared-mailboxes