Which of the following cannot be truncated in Salesforce?

In Salesforce, truncating refers to the process of quickly deleting all the data from an object while retaining its structure. However, not all objects can be truncated due to certain restrictions.

Custom objects referenced by another object cannot be truncated because doing so would violate data integrity. If other objects reference a custom object, truncating it would lead to orphaned records and broken relationships within the database.

Standard objects are also not eligible for truncation. Salesforce has predefined standard objects such as Accounts, Contacts, and Opportunities that have built-in functionality and relationships. Truncating these objects could lead to significant data loss and corruption, hence the restriction.

Additionally, custom objects with activated skinny tables cannot be truncated. Skinny tables are specialized tables that allow for improved performance by containing a subset of fields and records, but they come with specific considerations regarding data deletion and maintenance.

Given these constraints, it is correct to say that all of the mentioned object types cannot be truncated in Salesforce, confirming that option D is indeed the right choice.