The best measure of a star's brightness among the options provided is apparent magnitude. Apparent magnitude refers to how bright a star appears from Earth, taking into account its distance from us and any interstellar material that may dim its light.
Absolute magnitude, on the other hand, measures the intrinsic brightness of a star as if it were placed at a standard distance of 10 parsecs (about 32.6 light-years) from Earth. While it's a useful measure for comparing the actual luminosities of stars, it doesn't reflect how bright a star appears from our perspective.
Size and age of a star do not directly indicate its brightness, as size can vary widely among stars, and age does not determine brightness.