The correct answer would be D.
If you are breathless to the point where you are unable to talk during exercise then you are clearly pushing yourself too hard and overdoing it. You need to lower the intensity of the exercise level and work at a pace that suits your body. Eventually over time you will build up stamina and your energy levels and you will be able to exercise at high intensities. You should never be breathless during exercise as you run the risk of passing out. Everything should be done in moderation when you exercise especially when you first start out.