Which dynamic marking indicates a slightly louder volume than 'piano'?

The dynamic marking that indicates a slightly louder volume than 'piano' is mezzo forte. In musical terminology, 'piano' is used to instruct musicians to play softly, while 'mezzo forte' translates to "moderately loud." This marking allows for a balanced dynamic that is still soft but distinctly more prominent than 'piano.'

The other options represent different levels of loudness: 'forte' denotes loud playing, 'mezzo piano' suggests a volume that is between piano and piano (so softer than mezzo forte), and 'fortissimo' indicates a very loud volume. By understanding these distinctions, one can effectively interpret and execute dynamic markings in music performance.