In order to be a function, every value of the independent variable must be associated to only one value of the dependent variable.
If A is a function of G, that means G is the independent variable.
Looking at the table, the value G = 3 has two different values of A (A = 7 and A = 8), therefore A can't be a function of G (answer = NO).
If G is a function of A, that means A is the independent variable.
Looking at the table, the value A = 7 has two different values of G (G = 2 and G = 3), therefore G can't be a function of A (answer = NO).