Problem Solving

want to confirm my reasoning.....thanks all

The best way to solve these problems are by putting values

1) f(1)=1, f(2)= 4, f(1+2)=f(3) = 9 Not equal to f(1) + f(2)
2) f(1)=2, f(2)= 3, f(1+2)=f(3) = 4 Not equal to f(1) + f(2)
3) f(1)=1, f(4)= 2, f(1+4)=f(5) = 2.2 Not equal to f(1) + f(2)
4) f(1)=2, f(2)=1, f(1+2)=f(3)=2/3 Not equal to f(1) + f(2)
5) f(1)=-3, f(2)=-6, f(1+2)=f(3) = -9 EQUALS f(1) + f(2)

Since, 5 is the only one satisfying the condition.. Ans would be 5