CREB
The brightness of a star as seen from Earth is referred to as
brightness.

Explanation:

Astronomers define star brightness in terms of apparent magnitude. How bright the star appears from Earth and absolute magnitude is how bright the star appears at a standard distance of 32.6 light-years, or 10 parsecs. (A light-year is the distance light travels in one year is about 6 trillion miles, or 10 trillion kilo meters.) Astronomers also measure luminosity, the amount of energy (light) that a star emits from its surface.

Distance of a light year = 3600 x 24 x 365.25 x 3 x ≅ 10 trillion kilo meters

i'm assuming that the correct answer is indeed the first one/ "a"..