Answer
Graph f is impossible.
The entire mixture can not reach $0^{\circ}~C$, and then cool again below $0^{\circ}C$
Work Step by Step
Graph f is impossible.
The graph shows that the temperature of the ice increases until the entire mixture is $0^{\circ}~C$, but then the temperature of the mixture cools again below $0^{\circ}C$, which is impossible.