A discharge takes place between the two electrodes on applying the electric field to the gas in the discharge tube. The cause of this fluorescence was attributed to which of the following ?

the radiation which appeared to be coming from the cathode

In discharge tubes,  a high electric field is applied between the two electrodes the medium between them which is a gas at low pressure.  Due to the electric field, the gas gets ionized, releasing electrons which moves towards the anode thus constituting the electric current.  A low pressure is required as at high pressure the movement of electron will be retarded due to collision with nucleus / atoms of gas. 

The fluorescence was caused due to the radiations appeared to be coming from the cathode called cathode rays.