Is it an offence to obstruct clear vision of your number plates?

• A. Yes, at any time
  Explanation:

  "If you have a bike rack or towbar that covers the licence plate you must make other arrangements to display the plates clearly."
• B. Yes, but it is legal for a towbar or bicycle rack to cover the rear number plate
• C. Yes, but not if they are obscured by mud
• D. No, you are allowed to cover your number plates if you want to