According to the Salesforce documentation, ownership skew occurs when a large number of records (more than 10,000) are owned by a single user or queue. This can cause performance issues and lock contention when multiple users try to access or update those records. To prevent ownership skew, some of the recommended practices are:
Assign a default user as the owner of the records and do not assign any role to the default user (option C). This way, the records will not be visible to other users in the role hierarchy and will not cause sharing recalculations.
Keep users out of public groups that can be used as the source for sharing rules (option B).
Sharing rules based on public groups can cause excessive sharing calculations and lock contention when many records are owned by a single user or queue.
Assign a default user as the owner of the records and assign the top most role in the hierarchy to the default user (option E). This way, the records will be visible to all users in the role hierarchy, but will not cause sharing recalculations or lock contention.

To solve the issue of lack of governance in making changes to Salesforce, a data architect should recommend creating and managing a data dictionary and setting up a governance process for changes made to common objects. A data dictionary is a document that defines the metadata, structure, and relationship of each object and field in Salesforce. A governance process is a set of rules and procedures that govern how changes are proposed, reviewed, approved, and deployed in Salesforce. These measures will help NTO to maintain consistency, quality, and clarity of their data model and avoid confusion and errors among users.

Option A is correct because integrating Salesforce with the ERP environment is a technique to maintain data synchronization between systems. Option C is correct because utilizing an MDM strategy to outline a single source of truth is another technique to ensure data quality and consistency across systems.

Option D is correct because data flows should be reviewed with the business users to determine the system of record per object or field. The system of record is the authoritative source of data for a given entity or attribute. It may vary depending on the business context and requirements.

The two recommended options to assign Account ownership to mitigate potential performance problems are to let a "system user" own all the Account records without assigning any role to this user in Role Hierarchy, or to let a "system user" own the Account records and assign this user to the lowest-level role in the Role Hierarchy. This is because these options would reduce the number of sharing calculations and rules that need to be applied to the Account records, and improve the performance and scalability of the system. The other options are not recommended, as they would increase the sharing complexity and overhead, and potentially expose sensitive data to unauthorized users.