Why These Matter
Account Record Type (B)
• Person Accounts are a special type of Account. Salesforce requires you to have at least one existing Account record type before enabling Person Accounts, so that it can create the Person Account record type alongside it

Profiles Need Contact Read (A)
• Since a Person Account combines Account and Contact data, any profile that can read Accounts must also be able to read Contacts. Otherwise, users would see half the record

OWD Sharing Settings (D)
• Your org-wide defaults must be restrictive enough to let Salesforce enforce ownership on the merged Account–Contact record:

Either set Contacts to Controlled by Parent

Or set both Accounts and Contacts to Private
This ensures sharing rules work correctly when Person Accounts are enabled