It is defined as the frequency as the number that some event occurs. In this case, let us sum the frequency of each event (R, D, G, or I).
It is 7 Republicans, 7 Democrats, 1 Green Party member, and 5 Independents.
Now, to find the relative frequency, we use:

We know that the total number is 20, then we calculate:

Now, we can construct the following table: