The inverse square law states that the apparent brightness (B) of a star decreases with the square of the distance (d) : B∝(1)/(d^2) If a star's brightness at 1 parsec is 9 units, what would its brightness be at 3 parsecs?

Brightness varies inversely with the square of distance, so tripling the distance makes the brightness one-ninth as large. Starting from 9 units at 1 parsec, the brightness at 3 parsecs is 9 ÷ 3² = 9 ÷ 9 = 1 unit. The correct choice is A, 1 unit.

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