Rubber insulation deteriorates most rapidly when in contact with which substance?

Rubber insulation is particularly vulnerable to deterioration when it comes into contact with oil. This is due to the chemical composition of rubber, which can be compromised by various oils. When rubber is exposed to oil, it can swell, lose its mechanical properties, and eventually lead to cracking or failure of insulation. This degradation occurs because the oil interacts with the rubber's molecular structure, causing it to break down more quickly than it would from exposure to other substances.

In contrast, while water, ozone, and acid can also pose risks to rubber insulation, they do not tend to cause as rapid or severe deterioration as oil does. Ozone can lead to ozonolysis, which can weaken rubber, and water can cause issues like mold or degradation over time, but these processes are generally slower. Acids can corrode rubber but not at the same rate or intensity as oil. Thus, oil represents the most significant threat to rubber insulation's integrity, making it the correct choice.