which of the velocity component of projectile changes and other ...

[coco]

which of the velocity component of projectile changes and other remain constant and why.

[Millan]

Vertical component changes. Horizontal component doesn't.
The motion of object in the vertical direction is determined by act of gravity. So it has either acceleration or deceleration and hence velocity is continuously changing.
In case of horizontal component, there is no opposition to motion (neglecting air resistance) and hence there is nothing to impede the motion and hence it is constant.

[TI]

vertical velocity component of projectile changes as it is acted upon by gravity and horizontal component remains constant

[camila]

Consider the 2D case - you can split the projectile's velocity into 2 components  .. a 'vertical' (up & down) part and a horizontal (forwards) part ...

The 'up/down' (vertical) part will be effected by GRAVITY (which will pull it downwards until it hits the ground) .. whilst the 'forwards' (horizontal) part is only effected by air resistance (which is often ignored to make things simple).