Question

how can the water(not ice) is available at 0 degree celsius? explain.

Answer 1

Because the melting of ice is 0 degree in which it get converted into water

Answer 2

At 0 degree celsius, Water can be converted into ice by freezing and ice can be converted into water by melting.

So at 0 degree Celcius, water and ice can co-exist. It takes some time for the conversions to take place and in that time interval, both ice and water are present.