Which of the following was invented by Wilhelm Roentgen?

1. 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.

2. 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.

3. C

   Electric Bulb

   The practical incandescent electric bulb is associated with Edison, Swan and related inventors, not Roentgen. This option is therefore wrong.

4. 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.