Cathode rays are a beam of:

electrons

The correct answer is option 4. Electrons. 

Cathode rays are a beam of Electrons.

Cathode rays are streams of electrons observed in vacuum tubes, such as cathode ray tubes (CRTs). These rays were first discovered by J.J. Thomson in the late 19th century during experiments with evacuated glass tubes containing electrodes. Cathode rays originate from the cathode (negative electrode) and travel towards the anode (positive electrode). They are negatively charged particles, and their behavior can be influenced by electric and magnetic fields. Cathode rays played a crucial role in the discovery of the electron and contributed to the development of modern particle physics.