B

Step 1: Cathode rays are streams of electrons emitted from the cathode of a vacuum tube.
Step 2: The deflection of cathode rays by electric and magnetic fields indicates their particle nature. This is because charged particles, such as electrons, experience a force when they are placed in an electric or magnetic field, causing them to change direction.
Step 3: While the ability to propagate in vacuum (Option A) and producing fluorescence (Option C) may highlight some properties of cathode rays, they do not directly demonstrate their particle nature. Additionally, casting shadows (Option D) occurs due to any form of matter obstructing light and does not specifically pertain to their particle nature.
Therefore, the most compelling evidence for the particle nature of cathode rays is their deflection by electric and magnetic fields, making option B the correct choice.