It can be derived using Binomial theorem of probability

^{N}C_{i(}(p)^{i}(1-p)^{n-i }

Now 1 stations is sending with probability P ,and n-1 stations are **not** sending with probability 1-p

Now the probaility is= ^{n}C_{1} (p)^{1}(1-p)^{n-1}

=np(1-p)^{n-1}

so a is correct answer here