An example of kinetic energy continuously being changed to potential energy and back again might be
a. a girl swinging on a swing*
b. a train moving down a track
c. a plate sitting on the edge of a table
d. electric charges moving in a straight path

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.

i think the answer is either c or d

im thinking d might make more sense though

sorry if im wrong