The best measure of a star's brightness among the options provided is apparent magnitude.
Apparent magnitude measures how bright a star appears from Earth, taking into account its distance from us and any interstellar material that may dim its light. On the other hand, absolute magnitude measures the intrinsic brightness of a star as if it were placed at a standard distance of 10 parsecs from Earth. While both absolute and apparent magnitude are important for understanding a star's brightness, apparent magnitude specifically describes how we perceive it from our vantage point.
Age and size do not directly measure brightness.
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