Answer is given as B. I am also getting option A with exactly same logic. May be there is error in answer key.
Answer is right see h20 has an unusual property the density of ice is lower than that of water that's why it floats on the top of water so h20(l) has less volume as compared to h20(s) for the same mass so volume decreases so W>0 .
Thanks for correcting.
Here for w>0 is w the work done by system or work done on system
Since work done is +ve the work is done on the system