Proposed answer:
Strings are simply arrays of characters so the O-notation will be dependent on the number of characters in the string (if the loop depends on the length of the string). In this case the O-notation wouldn’t be affected because the length of the string is a constant.
Any other ideas? Am I reading this question correctly?
Try thinking about something that operates on an array of integers or an array of strings, clearly in the latter case you have an array of array of a primitive type rather than an array of a primitive type. How does this change things?