both a and b
When we talk about tuning on percussion instruments, we refer to the variety of notes and sounds produced by the instrument. There are two types: those of certain height and of undetermined height. The ones of determined height have bigger sound scale. They produce different sounds with different volumes and can be used as melodic or rhythmic elements, as most of these instruments already are.
Instruments of undetermined height, on the other hand, are those that generate a small variety of sounds and volumes, regardless of the strength of the touch and the frequency. Good examples of this type are castanets and the triangle. They are purely rhythmic instruments.
Some percussion instruments:áPlatesReco-recoBellTubular bellsTriangle