Answer
$.5$
Work Step by Step
We are interested in the event $E=${urban,pop/rock,tropical}
The sample size is $N=400$, of which $200$ were regional. The frequency of $E$ is $fr(E)=400-200=200$
The relative frequency of $E$ is $P(E)=\frac{fr(E)}{N}=\frac{200}{400}=.5$