The answer is the FORCE OF THE BALLS ACT IN PAIRS AND THE BLUE BALL PUSHES THE RED BALL BACK WITH OPPOSITE FORCE
EXPLANATION
This is a perfect example of newtons 3rd law "Every action has an equal and opposite reaction"when the red ball hits the blue ball it pushes into the ball and the ball sinks in to a point to absorb the energy and then the blue ball releases some of the energy back to the red ball pushing it away and the inertia makes the blue ball move away.
I hope this helps and can i get brainliest my dude :D thx