Correct Option: D

Scintillation or twinkling is generic terms for variations in apparent brightness or position of a distant luminous object viewed through a medium. If the object lies outside the Earth’s atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; within the atmosphere, the phenomenon is termed terrestrial scintillation. As one of the three principal factors governing astronomical seeing, atmospheric scintillation is defined as variations in illuminance only. Scintillation does not cause images of planets to flicker. Most scintillation effects are caused by anomalous refraction caused by small-scale fluctuations in air density usually related to temperature gradients.