Correct option is (d) 50 m
To explain I would say: Normally when an X-Ray is being taken, the people who are not supposed to be exposed to the radiations will be in a safe room where the radiation does not reach due to protective glass and other precautions. However, in case a potable X-Ray is being used and the X-Ray is being performed on the bedside, then the one who performs the X-Ray must wear a protective lead jacket and the other people must maintain a distance of minimum 50 m.