If the range of a random variable X is $\{0,1,2,3,4.......\}$ with $P(X =k)= \large\frac{(k+1)a}{3^k}$ for $k \leq 0,$ then a is equal to :

Question

$$\begin {array} {1 1} (a)\;\frac{2}{3} & \quad (b)\;\frac{4}{9} \\ (c)\;\frac{8}{27} & \quad (d)\;\frac{16}{81} \end {array}$$