Answer:
a. bending of light rays by turbulent layers in the atmosphere, variable absorption by interstellar gas along the line of sight to the star.
Explanation:
Light is refracted as it passes from one media to another, that same scenario is what causes that stars twinkle. When the light coming from a distant star reaches the Earth atmosphere, the beams of light will be refracted over and over as a consequence of the different temperature and density on the layers of the atmosphere, getting as result a zigzag pattern (twinkle).
Another cause of twinkle is the interstellar gas. The light will be absorbed in a variable way along the line of sight between the star and the Earth by different lumps in the insterstellar gas.
