the very best atmospheric conditions on earth result in an angular resolution limit of about 0.4 arcseconds if an astronomy observes under such conditions at visible wavelengths using a 0.2-meter telescope, the angular resolution is limited by

ATurbulance in the earths atmosphere

BDiffraction by dust in the interstellar medium

C)Variable changes in the physical size of the star being observed

D)Diffraction effects from the telescopes optics