The correct answer is option 3, which states that the heating of big transformers is due to both current producing heat and hysteresis loss occurring in the transformers.

Let`s analyze each option to better understand why the correct answer is 3:

Option 1: This option suggests that heating is solely caused by current producing heat in the transformers. While it is true that electrical currents generate heat, it is not the only factor responsible for the heating of transformers. Therefore, option 1 is incorrect.

Option 2: This option suggests that heating is solely due to hysteresis loss occurring in the transformers. Hysteresis loss is the energy dissipated as heat when the magnetic domains within the transformer core continually realign with the alternating current. While hysteresis loss does contribute to the heating of transformers, it is not the only factor. Additionally, the liquid used for cooling does not play a role in this specific phenomenon. Therefore, option 2 is incorrect.

Option 3: This option correctly identifies both current producing heat and hysteresis loss as causes of heating in transformers. As mentioned earlier, electrical current generates heat, and hysteresis loss adds to this heat production. Therefore, option 3 is the correct answer.

Option