i give it a try:
String c [] ={"1.0", "10.0", "20.0", "30.0", "40.0", "50.0", "60.0","70.0", "80.0", "90.0","100.0", "200.0", "300.0", "400.0","500.0", "600.0", "700.0", "800.0", "1000.0", "2000.0"};
for (i = 0; i < c.length; i++){
String opt = String.format("-C %S 1.0 -L 0.001 -P 1.0E-12 -N 0 -V -1 -W 1 -K weka.classifiers.functions.supportVector.PolyKernel -C 250007 -E 1.0 ",c[i]);
}
i used String.format("", ...) function and added %S as placeholder where the value of c[i] will be put into.