Final answer:
When Crookes placed a solid object inside the cathode ray tube, it cast a shadow, proving that cathode rays consist of particles with mass. So the correct option is a.
Step-by-step explanation:
When Crookes placed a solid object inside the cathode ray tube, the object casts a shadow on the other end of the tube. This experiment by Sir William Crookes involved a cathode ray tube with a small metal rail and a paddle wheel between the electrodes. As the cathode ray tube was operated, the paddle wheel rotated, proving that the cathode rays had mass since they moved an object. The shadow observed on the glass serves as proof that the cathode rays consist of particles: the object was blocking these particles from reaching the anode, creating the shadow. Through his work, Crookes significantly contributed to the discovery that cathode rays are streams of electrons which led to later technological advancements such as television and the oscilloscope.