Explanation
Statement A is incorrect: The distance between stellar bodies does change, but often very slowly or in predictable orbits. Planets orbit stars, stars orbit galactic centers, and galaxies orbit each other, all causing continuous changes in distance. And this statement is not related to why distance is measured in light years.
Statement B is incorrect: While the gravitational constant is fundamental, the effective gravity of stellar bodies changes due to shifts in their mass (e.g., fusion, stellar winds), changes in their radius/size (expanding into red giants, collapsing into white dwarfs/neutron stars), and changes in the distance between them as they orbit. And this statement is not related to why distance is measured in light years.
Statement C is incorrect: Though mostly light travels in a straight line, it can be bent by Gravity- phenomenon known as Gravitational Lensing.
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.