$\Rightarrow Propagation Time =$$\frac{Distance}{Speed}=\frac{2\,Km}{2 * 10^{8}\, m/s} = 10\, \mu s$
Including the monitoring station there are 10 stations.
Now once the token is released by the monitoring station it should wait atleast the time till the token comes back after travelling 2 Km through other 9 stations present in the ring.
So minimum time to wait is,
$10 \, \mu s$ propagation time for $2\, Km$ distance $+$ $9$ stations hold the token for $2 \mu s$ each.
$\Rightarrow 10 \, \mu s + 2\, \mu s * 9 \, stations \Rightarrow 28 microseconds$
Anyways gate answer key has accepted 30 also as the answer. So instead of 9 if you put 10, it was still accepted.