Final answer:
The ball in the bus is also moving in a straight line with constant velocity, following Newton's first law of motion, since there is no external force disturbing its motion.
Step-by-step explanation:
When Ellie is playing with a ball in a bus that moves in a straight line with constant velocity, the ball is also moving in a straight line with constant velocity. This is due to Newton's first law of motion which states that an object will remain at rest or in uniform motion in a straight line unless acted upon by an external force. In this scenario, since the bus is moving at a constant velocity and there is no external force acting on the ball, the ball moves with the bus at the same constant velocity. Hence, the correct answer to the student's question is option 1).