B & C

According to sf documentation::
If you do have a compelling reason for assigning ownership to a small number of users, you can minimize possible performance impacts by not assigning the user(s) to a role.

If the user(s) must have a role to share data, we recommend that you:Place them in a separate role at the top of the hierarchyNot move them out of that top-level roleKeep them out of public groups that could be used as the source for sharing rules
Ans. B,C

Correct answer is B and C.
check the link for refrence: https://developer.salesforce.com/docs/atlas.en-us.draes.meta/draes/draes_group_membership_data_skew.htm

I'm sure that the answer A is wrong
A. Let a "system user" own the Account records and assign this user to the lowest-level role in the Role Hierarchy.
The answers should be B and C
B. Let a "system user" own all the Account records and make this user part of the highest-level role in the Role Hierarchy.
C. Let a "system user" own all the Account records without assigning any role to this user in Role Hierarchy.

The "A" response is wrong