Answer:
d. it is a description in nature which cannot remain unchanged.
Step-by-step explanation:
The force gravity refers to the force due to the acceleration of gravity, which is towards the nucleus of the Earth.
Now, it's known that the force of gravity pulls all objects downwards. But, this is relative, because depending on the point of reference downwards can also refers to upwards. In other words, if you consider it from the space, you will see deduct that there's no directions at all, in fact, there you would realize that gravity is just an atraction force, not just a downward force.
That's why the right answer here is d., because the force of gravity is a description of the nature, because a truly explanation of the matter is not clear yet (modern physyics is looking for that), and "cannot remain unchanged", because the downwards direction depends on where we at.