13. a basic observation of a star is how bright it appears. this brightness is known as the star's a. absent magnitude. b. apparent magnitude. c. absolute magnitude. d. luminosity.

Star brightness is defined by either 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). 

Type in the minus sign