Choice "D" is correct. Denial of usual trade credit is one example of a financial difficulty that may be indicative of substantial doubt about an entity's ability to continue as a going concern.
Choice "a" is incorrect. Pervasive significant related party transactions, by themselves, are not necessarily indicative of an entity's ability to continue as a going concern.
Choice "b" is incorrect. The ability to pay arrearages in preferred stock dividends would be indicative of financial health and would not create substantial doubt about an entity's ability to continue as a going concern.
Choice "c" is incorrect. Restrictions on the disposal of principal assets would not cause an auditor to have substantial doubt about an entity's ability to continue as a going concern.