Given below are two statements. One is labelled as Assertion (A) and the other is labelled as reason (R). 

Electric force is applied on an electron when it is moving in an electric field whether it is moving parallel to the electric field or not.  When it is moving parallel, then the electric force will be parallel to the motion, so, the motion will be always parallel to the electric field, otherwise it will deviate from its path. When a charge particle is moving parallel to a magnetic field, the magnetic force is zero on the charge, as,

$\left|\overrightarrow{{\mathrm{F}}_{\mathrm{m}}}\right|=\left|\mathrm{q}\left(\overrightarrow{\mathrm{v}}\times \overrightarrow{\mathrm{B}}\right)\right|=\mathrm{qvB} \sin 0^0=0$

Electron beam will be undeflected when the net force on the electrons is zero.

$\overrightarrow{{\mathrm{F}}_{\mathrm{e}}}+\overrightarrow{{\mathrm{F}}_{\mathrm{m}}}=0$

$\Rightarrow \mathrm{eE}=\mathrm{evB}\Rightarrow \mathrm{v}=\frac{\mathrm{E}}{\mathrm{B}}$

In this case the fields are mutually perpendicular with the velocity of the electrons.