The primary form of evidence that has led astronomers to conclude that the expansion of the universe is accelerating is by the observation of white dwarf supernovae.
Answer: D
Explanation
The Universe is becoming larger in an accelerated rate which means the galaxies are stretching away from the observer with increasing acceleration.
The speed with which the galaxies in the universe are receding away from the observer is varying with time.
This process is termed as accelerating expansion of universe.
In 1998, 2 independent teams first observed this. The first evidence for accelerating expansion of universe was obtained by observing the red shift of white dwarf supernovae named Ia supernovae.