The instrumental solo pieces can be monophonic or polyphonic.
Monophonic instruments, like those of the brass and wind sections, can only produce one single note at a time, and so can only perform monophonic pieces.
Polyphonic instruments, such as the guitar, piano, and harp, can play multiple notes at once, and so can play both monophonic and polyphonic pieces.
Bowed string instruments, such as the violin, viola, cello, and double bass, are capable of playing polyphony, but aren't capable of playing triads, or complete chords.
Despite this, many composers such as Bach or Paganini have written polyphonic solo music for bowed string instruments, though these are very difficult to perform.