Lets see if we can annotate this a bit:
public String chooseOne(ArrayList<String> choices) {
Random rand = new Random();
ArrayList<String> choices;
return rand.nextString(ArrayList<String> choices);
}
Given that
ArrayList.size()
returns the number of elements in the list and
rand.NextInt(int bound)
returns the next integer in the range (0 .. bound), you should be able to figure how to fix your code.