The physical quantity that remains unchanged during the flight of an oblique projectile is the horizontal component of velocity.

Here's why:

* Neglecting Air Resistance: In ideal projectile motion, we assume no air resistance. This means there's no horizontal force acting on the projectile.

* Newton's First Law: Newton's First Law of Motion states that an object at rest stays at rest and an object in motion stays in motion at a constant velocity unless acted upon by an unbalanced force. Since there's no horizontal force, the horizontal velocity remains constant.

Important Note: In real-world scenarios, air resistance plays a significant role, and the horizontal velocity will decrease over time. However, the concept of constant horizontal velocity is a fundamental approximation used in projectile motion analysis.