Option A is good practice from usability point of view,if you are using window xp or mac. when a hardware is not available, then options related that hardware is often time disabled. so its good practice to keep them disabled when its hardware is not available.
there is an another option with cross mark, few high profile website is using this option for better user experiences and instructions.
See the below option, you can enable the camera with cross mark. and when user click on video icon so there should be some information like camera not found, you should install it from (some where location).
I use rough images for explanation in this answer, you can replace these images with your iphone images.