How Must an Object Be Moving for Us to Be

How must an object be moving for us to be able to use the theory of special relativity to describe the object?

A) The object must be moving close to the speed of light; how speed and direction change is not important.
B) The object must be moving at a constant speed in a straight line; how fast it is moving is not important.
C) The object must be moving in a constant direction; how its speed changes is unimportant.
D) The object must be moving at a constant speed; whether the direction of motion changes is unimportant.