Answer:
It depends on each instrument. Strings and some band instruments such as Trombone, tuba, and flute are in the key of C so you don't have to transpose them. Other instruments like Clarinet and trumpet are in the key of B flat so if you want them to play a C you have to tell them to play a D because everything they play sounds a A whole note below what you tell them to play. Not all band instruments are in the key of B flat thought. French horn and English horn for example are in the key of F. There are many different kinds of instruments and different keys they are in.