C. No question (they don't do a bass clef version). For the sake of completion, the reason other keys are available is for what are called 'transposing instruments'. In short, some horns play a whole tone higher than concert so when you play a C, they would play a concert D so, in order for it all to work, the sheet music is transposed down a whole tone to Bb so, when they play a note 'as written', it sounds a whole tone lower than the note would normally indicate were it not a transposing instrument. Other instruments are a minor third lower so, in order for it to work, the music is transposed [i]up[/i] a minor third to Eb. There are other transposing instrument but, in popular music, C (treble and bass clef), Bb and Eb usually covers it. What came as a revelation to me as I became more and more familiar with writing for horns (I polay a saxophone but only at a VERY rudimentary level), was that reading a Bb clef and and Eb clef does NOT require new learning like, say, a treble clef and a bass clef would. A C on a Bb chart requires the saxophonist to put his or her fingers on the same notes which they would a C on an Eb chart. The chart does the transposition, not the horn player. It's all quite simple really, it just sounds complicated.