An electron is moving along positive x-axis. A uniform electric field exists towards negatively y-axis. What should be the direction of magnetic field of suitable magnitude so that net force on electron is zero.

negative z-axis

$\text{Since Electric field is along negative-y axis hence force on electron is along positive-y axis.}$

$\text{For net force to be zero magnetic force should be along negative-y axis.}$

$ -e \vec{v}\times \vec{B}\text{ should be along negative -y axis } \Rightarrow \vec{v}\times \vec{B} \text{ should be along positive y-axis}$

$\text{magnetic field should be along negative-z axis.}$