In an M/M/1 queuing system, the number of arrivals in an interval of length T is a Poisson random variable (i.e., the probability of there being n arrivals in an interval of length T is $\frac{e^{\lambda T\left ( \lambda T \right )^{n}}}{n{}'}$ 
The probability density function f(t) of the inter-arrival time is given by
(a) 
$\lambda ^{2}\left ( e^{-\lambda ^{2}t} \right )$ (b) $\left ( \frac{ e^{-\lambda ^{2}t}}{\lambda ^{2}} \right )$ 
(c) $\lambda e^{-\lambda t}$ (d) $\left ( \frac{ e^{-\lambda t}}{\lambda} \right )$