Twinkling of stars (scintillation) is caused by atmospheric refraction. - Light from distant stars travels through the Earth's atmosphere before reaching our eyes. - The atmosphere is not uniform; it consists of layers with varying temperatures and densities. - Variations in temperature and density cause variations in the refractive index of the air. - As light from a star passes through these turbulent layers with changing refractive index, it undergoes continuous refraction in random directions. - This causes fluctuations in the apparent position and brightness of the star as seen from Earth. These rapid fluctuations are perceived as twinkling. - Planets, being much closer, appear as extended sources of light rather than point sources. The light from different parts of a planet's disc undergoes similar but independent variations, which average out, so planets do not twinkle noticeably.Option A correctly identifies the cause: changes in atmospheric refractive index due to temperature variations (and hence density variations) lead to varying refraction of starlight. Option B is incorrect; stars emit light continuously. Option C is incorrect; twinkling is an effect on light from a single star due to atmospheric effects, not interference from different stars. Option D is incorrect; while diffraction occurs, twinkling is primarily an effect of refraction due to atmospheric turbulence.