An object is in motion when
a.
you observe the object move.
b.
the object’s speed increases.
c.
the object’s displacement is greater than the distance traveled.
d.
the object changes position relative to a stationary reference point.

The correct answer is D. The object changes position relative to a stationary reference point.

Explanation:

Motion occurs when a specific body/object displaces or changes its position which implies the object moves from a reference point to a new one. This is usually explained as forces act over the object and make the object move or change from a stationary state to motion.

In physics, motion is widely studied according to scientific laws and this is related to main other concepts such as speed, velocity, direction, acceleration or inertia that as motion are commonly studied based on a reference system or point. According to this, it can be concluded an object is in motion if "the object changes position relative to a stationary reference point".

half of a circle

explanation:

the arrow and the top of the circle is matching together. it is not a polygon because a polygon would always be the same shape. therefore, an half of a circle is called semicircle so you would find the radius of it.

a

explanation:

because they are cartillidge