The primary reason professional observatories are built on the highest mountaintops is to (a) get away from city lights; (b) be above the rain clouds; (c) reduce atmospheric blurring; (d) improve chromatic aberration.

(c) reduce atmospheric blurring

Explanation:

Observatories are the locations used to study the celestial events, climate, astronomy etc. As the observations are made with the radio and visible light of the electromagnetic radiations. So in order to have the clear view, observatories are made at higher places far from population, in the dark areas  having clear air, where they can clearly observe the sky. Atmospheric blurring is caused by the atmospheric turbulence. That cause the light coming from the stars to get slightly refracted thus effecting the results of the telescopes. In order to overcome it observatories are made at specific places like on the highest mountaintops.

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