Which of the following is a sign of overheating in electronic components?

Blackened or scorched components serve as a clear indicator of overheating in electronic components. This physical damage often occurs as a result of excessive heat that can break down materials and lead to failure. When components overheat, they can reach temperatures high enough to char the insulation around wires or the components themselves, leading to visible signs of burning or scorching. This is a direct result of overheating rather than a symptom of another issue, thus making it a definitive sign of thermal damage.

Other symptoms, while they may indicate issues, do not specifically signify overheating. Unstable voltage readings may suggest a variety of problems such as fluctuations in power supply or component malfunctions unrelated to heat. Cold solder joints can cause connectivity issues but are typically a result of improper soldering rather than overheating. Dust accumulation, while it can trap heat and contribute to overheating in a circuit, is not a direct indication that any specific component has overheated; rather, it suggests a need for maintenance or cleaning. Thus, the clear and observable physical signs of damage to components make blackened or scorched components the correct choice when identifying overheating.