Using equipment that you haven't been trained on a. is ok as long as you're careful and read the directions. b. allows you the opportunity to teach yourself how to do a particular exercise. c. can cause major damage to your body and to the equipment. d. is a good way to add variety to your workout.

By "equipment" here, I would assume that you are talking about gym equipment. While it is safe to try out a new equipment for as long as you practice caution and follow the instructions (a), it can probably damage your body in the end especially if you are following a weight program. If you are not careful, you might also damage the equipment and not just your body. So, in this case, I would say (c).

Answerbarrel vault