Star brightness is defined by either <span>D. apparent magnitude - when we measure it from Earth (that is how bright it is from Earth) - this is the closest to "basic observation" and the best answer
or by absolute magnitute - how bright it would be at a certain distance (this allows to compare the brightness).
The brightness of the star is measured by the size, which is distinguished in apparent and absolute. The apparent magnitude measures the brightness of the star perceived by the observer; it therefore depends on the real brightness of the star, the distance of the Earth and the atmosphere of the Earth has caused alterations (the view).
The absolute magnitude It is the magnitude that the star would have if it were at the distance of 10 parsecs (or 32.6 light years) from Earth (and it is closely related to the real brightness of the star).
