Venus's approximate greatest apparent magnitude as seen from Earth is −4.5.
Apparent magnitude measures how bright an object looks from Earth, and lower, more-negative values mean greater brightness. Venus reaches this peak when it is an intermediate crescent: close enough to Earth to appear large, but still showing enough sunlit surface to reflect substantial light.
The brightest moment is therefore not when Venus is full. At full phase, Venus is on the far side of the Sun and appears smaller; near inferior conjunction, it is closer but only a very thin crescent. The maximum occurs when the illuminated portion has its greatest apparent area, roughly when Venus is about 28% illuminated.
Venus's phases were famously observed telescopically by Galileo Galilei in 1610. Those changing phases helped undermine the Ptolemaic Earth-centered model because a full set of phases is naturally explained when Venus orbits the Sun. Modern references may quote a more precise peak near −4.9, but −4.5 is the approximate value used by the cited Venus-phase reference and is the intended quiz answer.