Which of the following is a common indicator of poor fuel quality?

The option indicating a strong burnt fuel smell is a valid indicator of poor fuel quality. When fuel degrades or contains impurities, it can produce a distinctive burnt odor, which is often associated with incomplete combustion or the presence of contaminants. This smell can indicate that the fuel is not burning effectively, which can lead to engine performance issues, increased emissions, and potential damage to the engine components.

High cetane number, on the other hand, is a characteristic of good quality diesel fuel, as it indicates better ignition properties and more efficient combustion. A low flash point could signal high volatility which might impair performance or present safety risks, but it does not inherently indicate that the fuel is of poor quality. High viscosity can affect fuel flow and atomization, but by itself, it does not directly reflect the quality of fuel in terms of its combustion characteristics. In this context, the burnt fuel smell stands out as a clear sign of degraded fuel quality.