Final answer:
Cash basis accounting is the methodology that maintains the original principal of trust by only recording transactions when cash changes hands, keeping the principal untouched by unrealized revenues and expenses.
Step-by-step explanation:
The basis of accounting that best maintains the original principal of a trust is cash-based accounting. This method only recognizes revenue and expenses when they are received or paid, thus maintaining the accuracy of the principal amount since unrealized revenues and expenses are not recorded. The other methods, legal basis, accrual basis, and modified accrual basis, involve recognizing revenues and expenditures at different times, which can alter the appearance of the principal balance due to the recognition of items not yet realized in cash.