Proper use of the helmet means that the retention strap...
A)must not be fastened.
B)may be worn unfastened on urban roads.
C)must be properly fastened so that the helmet effectively fulfils its function.Correct
The helmet must fit well on the head, it should not be worn tilted backwards or forwards over the eyes, and the safety strap must be fastened.
The safest helmet is the full-face type, which covers the entire face.