Is Zero Celsius is the coldest possible temperature.

No. In news reports in the winter, temperatures in britain can go to the negative Celcius. The lowest recorded naturally occuring temperature on Eart, in Antarctica, was about - 73C. The coldest temperate is 0 Kelvin, also known as absolute zero. It's equivalent of about - 273C

No, it is not because it simply freezes water but absolute zero is zero kelvin or - 273 degrees Celsius