The suggested answer is B.
Set the previous version databases to be read-only
If you want your original environment to remain available to users in a read-only state, set the databases to read-only before you back them up.
Complete this step for each content database in your environment. Depending on your organization, you might need a database administrator to complete this step.
Incorrect:
Not A: The content databases, not the server instance, should be set to Read-only.
Note:
* To set content databases to be read-only
1. Verify that you have the following administrative credentials: You must be a member of the db_ownerfixed database role in each database.
2. Open SQL Server Management Studio.
3. Right-click the content database that you want to change to read-only, and then click Properties.
4. Select the Options page, and, in the Other options list, scroll to the State section.
5. In the Database Read-Only row, click the arrow next to False, select True, and then click OK.
6. Repeat for all other content databases.
* A farm is considered to be read-only if any of the following are true:
/All content databases are set to read-only.
/Service application databases are set to read-only.