A positively charged particle moving along x-axis with a certain velocity enters a uniform electric field directed along positive y-axis. Its :

The correct option is B Vertical velocity changes but horizontal velocity remains constant.

The force on the positive charge moving along $x-axis$ is given as

$F=qE$

The positive charge will move along the $x-axis$ because of the field lines which originates from positive and terminates on a negative charge. But when the charge enters in the field along $y-axis$ then the vertical component will change and the horizontal (along $x-axis$ ) will remain same.