Universal Containers uses a private sharing model on Accounts.

The three reasons user A has access but user B is unable to access the record are:

User A is on the same account team as user C. Account teams are groups of users who work together on an account. Users who are on an account team can have access to the account and related records owned by other team members.

User C has manually shared the record with user A. Manual sharing is a way of granting access to individual users or groups by the record owner or someone above them in the role hierarchy.

User A is in a public group that has access via a sharing rule. Public groups are groups of users who can be used to share data or assign permissions. Sharing rules are automatic rules that grant access to groups of users based on certain criteria. Option B and D are not reasons for user A’s access.