Statement A is incorrect: as we know the universe is ever expanding so the distance between two stellar bodies does not change.

Statement B is incorrect: as the Gravitational power is dependent on the mass of the object, it changes with a change in mass.

Statement C is incorrect: Though mostly light travels in a straight line, it can be bent by Gravity.

Statement D is correct: the speed of light does not change in a vacuum. So, it is a constant that can be used for measurement.