Final answer:
To discover if LaRue took out a mortgage for a property purchase, the most relevant source would be the county real property records. These public records contain specific information about mortgages on properties.
Step-by-step explanation:
If McCloud, an internal investigator for a private company, is looking to uncover information regarding whether LaRue, the company's controller suspected of embezzlement, took out a mortgage on a recently purchased mansion, the most direct source of this information would be b. County real property records. These records are public and typically include details like ownership history, mortgages, and liens against the property. While LaRue's income tax return and bank account transactions might offer indirect clues, they would not definitively reveal a mortgage's existence on the property as the real property records would.