B

Cathode rays are streams of particles that are emitted from the cathode in a vacuum tube.
Step 1: Understand the definition of cathode rays: They are generated when electrons are emitted from the cathode and travel through the vacuum towards the anode.
Step 2: Recognize that cathode rays are composed of negatively charged particles, which are indeed electrons.
Step 3: Review the options:
- Option A (Photons) are particles of light and not related to cathode rays.
- Option C (Protons) are positively charged particles found in atomic nuclei.
- Option D (α-particles) are helium nuclei and do not constitute cathode rays.
Therefore, the correct answer is Option B: Electrons.