Random name-picker with array
First of all, you do not need to create an instance of Random class every time.

Then, you may use the ternary operator (the ?: operator) to select the array from which you want to take the element.

And, assuming that you have to take a random element form the array, you may use just a[T.Next(a.Length).

Random T = new Random();
. . .
string[] a = radioButton1.Checked? array.top:
             radioButton2.Checked? array.jung:
             radioButton3.Checked? array.mid:
             radioButton4.Checked? array.marksmen:
             radioButton5.Checked? array.supp:
             null;
if (a != null)
    textBox1.Text = a[T.Next(a.Length)];