Keith Williams is correct. The only feasible answer provided in the multiple choice is Vmo. A barbers pole does not INDICATE temperature or altitude, it only INDICATES a speed limit. The question is worded stupidly. Had it asked what a barber's pole USES to provide its readings, then options A or B could possibly be considered (I don't know, I can't be bothered to dig out the books).
Are you absolutely sure this question is worded correctly in your post?