A transformer is often filled with oil. The oil used should have.
high dielectric strength
The explanation for the correct option(s):
Option B: high dielectric strength.
Explanation for the incorrect option(s):
Option A: low viscosity.
Good oil should have a low viscosity so that it offers less resistance to the conventional flow of oil and does not affect the cooling of a transformer, but low viscosity oil affects the cooling of the transformer.
Option C: Low boiling point.
At a low boiling point transformer, oil stops convention from flowing and obstructs cooling in the transformer.
Option D: high thermal conducting
Transformer oil or insulating oil is an oil that is stable at high temperatures and has excellent electrical insulating properties so it has no property of high thermal conducting.
Hence, a transformer that is often filled with oil should have high dielectric strength.