When ice and steam are mixed together, they undergo a process called fusion. During fusion, the ice absorbs heat from the steam and starts melting. The temperature of the ice remains at 0°C until it completely melts and converts into water.

Once all the ice has melted, the water starts to absorb more heat and gradually increases in temperature. The steam, on the other hand, starts to condense into water as it loses heat.

During this process, the temperature of the mixture remains constant at 0°C until all the ice has melted. After all the ice has melted, the temperature of the mixture will start increasing.

Since the given options are in degrees Celsius, we can eliminate options 1 and 2 as the temperature cannot remain at 0°C after the ice has melted.

Option 3 of 80°C is too high, as the mixture will not reach such a high temperature after the ice has melted.

Therefore, the correct answer is option 4, which states that the temperature of the mixture will be 100°C after the ice has melted completely and reached thermal equilibrium.