Explanation
- A
Radio
Radio as a practical wireless system is associated with Marconi and with earlier work by Hertz, Bose and others, not with Roentgen. This option is not the key.
- B
X-Ray Machine
Wilhelm Conrad Röntgen discovered X-rays in 1895 in Würzburg and produced the first medical radiographs. The X-ray machine rests on that discovery. This option is the official key.
- C
Electric Bulb
The practical incandescent electric bulb is associated with Edison, Swan and related inventors, not Roentgen. This option is therefore wrong.
- D
None of the above
Because the X-ray machine is correctly attributed to Roentgen’s discovery, none of the above is unnecessary. This option is not the key.
Summary. Official key is (b) X-Ray Machine. Röntgen’s 1895 discovery of X-rays founded radiography and the diagnostic X-ray apparatus. Radio and the electric bulb belong to other inventors. The stem asks what he invented among the list. Only (b) is his.