Final answer:
Usual trade credit from suppliers being denied is most likely to cause an auditor substantial doubt about an entity's ability to continue as a going concern, as it indicates potential cash flow issues and financial instability. Hence, the correct answer is option (b).
Step-by-step explanation:
Out of the conditions or events listed, usual trade credit from suppliers being denied is most likely to cause an auditor to have substantial doubt about an entity's ability to continue as a going concern. When suppliers refuse to extend credit, it suggests a lack of confidence in the company's financial stability or its ability to make timely payments, which could be indicative of cash flow problems. This situation can lead to further financial strain, as the company may have to seek alternative and potentially more expensive forms of financing to maintain operations, or could face disruptions in its supply chain.
In contrast, significant related party transactions might raise questions of propriety but would not necessarily indicate immediate financial distress. Restrictions on the disposal of principal assets might limit the company's ability to raise funds in the short term, but do not directly suggest that the company cannot meet its ongoing obligations.