Which musical instrument is characterized by metal bars of varying lengths that are struck with mallets?

The instrument characterized by metal bars of varying lengths that are struck with mallets is best represented by the xylophone. The xylophone consists of wooden or metal bars that produce different pitches based on their length when struck. While the marimba is quite similar and also uses bars struck with mallets, it typically features wooden bars and has resonators that enhance its sound, which differentiates it from the xylophone.

The piano, on the other hand, is a keyboard instrument with strings struck by hammers, rather than being based primarily on tuning bars. Drums produce sound through membranes that are struck and do not involve bars; instead, they typically generate rhythm and volume rather than pitch variation. Thus, the xylophone is the correct choice, as it directly matches the description of using metal bars that are struck to create sound.