An atom of an element can change into an atom of another element if it is A
extremely stable and has a long life.
B
radioactive and its nucleus emits or traps particles.
С
an isotope and it contains more electrons than neutrons.
D
an ion and its atomic number is larger than its mass number.

I would say d because it’s reasonable

You need to be more specific.  i really don't know what a "motion map" is.you can draw a graph of an object's position, distance, displacement, speed,velocity, or acceleration versus time, or you can draw a graph of the object's x-position versus its y-position and not give any information about time at all.if the graph presents position or displacement against time,  then constant velocity is a straight line . . i.e. a line that has  constant slope.if the graph shows speed against time, then constant velocityis a horizontal piece of line.if the graph shows acceleration against time, then constant  velocity is a piece of line that lies on the x-axis . . it's value is zero.