Which of the following can cause a fire in connection with flammable solids?

Excessive humidity and moisture absorption can indeed lead to fires when dealing with flammable solids. When certain flammable solids absorb moisture, it can create a situation where the material becomes more reactive or potentially hazardous. The presence of moisture can alter the physical and chemical properties of flammable solids, making them more likely to ignite under the right conditions, such as when exposed to an ignition source like a spark or flame.

Additionally, moisture can contribute to the formation of other harmful substances or reactions that might not occur in dry conditions. For example, some flammable solids can generate heat when they absorb moisture, potentially leading to self-ignition. Therefore, maintaining dryness and controlling humidity is crucial in environments where flammable solids are stored or used.

In contrast, the other options, while they may have certain effects, do not directly relate to fire hazards in the same manner that excessive moisture does with flammable solids. Prolonged exposure to sunlight, high-pressure storage, and low ambient temperatures do not typically create the same level of risk for ignition in flammable solids compared to the direct influence of moisture.