The correct answer is: D. 150°C
Transformer oil is a dielectric fluid used to insulate and cool the windings of a transformer. It is a non-flammable, non-toxic liquid that is typically made from mineral oil or synthetic oil.
The oil temperature in a transformer is typically kept below 150°C to prevent the oil from breaking down and releasing gas. If the oil temperature exceeds 150°C, the oil can start to decompose and release gas. This gas can cause arcing and sparking, which can lead to a fire or explosion.
Therefore, gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds 150°C.
Option A: 50°C is too low a temperature to cause the oil to decompose.
Option B: 80°C is a possible temperature at which the oil could start to decompose, but it is not the most likely temperature.
Option C: 100°C is a more likely temperature at which the oil could start to decompose, but it is still not the most likely temperature.
Option D: 150°C is the most likely temperature at which the oil will start to decompose and release gas.