Answer (B) 2/3
Explanation: the multiplicative rate is the ratio of the values y(k)/y(k+1) (with k being the index). This ratio must be the same for the function to be exponential. We can take y(2)/y(1) = 4/6 = 2/3.
We can use 2/3 to verify the remaining entries:
x=3: 4*2/3 = 8/3 = y(3)
x=4: (8/3)*(2/3)=16/9=y(3)
