Both options, B and D, keep the finance department's data on-premises to ensure security and compliance. However, they have slight differences:
B: Focuses on migrating all other SharePoint data and Exchange mailboxes to Microsoft 365 while specifically addressing the finance department's SharePoint data only.
D: Specifies leaving both SharePoint data and mailboxes for finance department users on-premises.
In this context, D is a better choice because it explicitly maintains the security of both the SharePoint data and the mailboxes for the finance department. This option comprehensively addresses potential security and compliance risks while allowing for a broader migration of other services to Microsoft 365.
So, in my opinion, the best option is D.