The question asks which of the following invocations are valid. Option B is valid because the find() method is a built-in string method in Python that searches for a specified substring within the string. Option C is also valid because the rindex() method is another built-in string method that returns the highest index of a specified substring within the string. Options A and D are invalid: Option A attempts to use a sort function incorrectly on a string, and Option D incorrectly tries to use the sorted() method on a string, which should be used on iterables like lists, but not strings.