Let the stations be $S_1, S_2, S_3,...,S_n $
The probability that only one station transmits is:-
Either $S_1$ transmits and no other transmits
Or, $S_2$ transmits and no other transmits
Or, $S_3$ transmits and no other transmits
so on.
$p(1-p)^{n-1}+p(1-p)^{n-1}+p(1-p)^{n-1}+...$
=> $np(1-p)^{n-1}$
Option A