The line of sight is kept as high above ground surface as possible to minimise the error in the observed angles due to

ANS:D - both shimmering and horizontal refraction.

Keeping the line of sight high above the ground surface is essential to minimize the error in the observed angles due to shimmering, horizontal refraction, and vertical refraction. So, all the given option is the correct answer. Shimmering shine with a soft, slightly wavering light.