If the question is about whether you have a transposing instrument (or not) your answer would be 'I'm in C' or 'I'm concert pitch'. Your soprano sax playing friend would answer 'B♭'. This might occur if someone's handing out sheet music and needs to know whether to give you a transposed copy.

If the question is about what key you prefer to play a song in (or about what key you're already playing it in!) you'd answer 'G', 'D', 'A minor' or whatever it was. You might clarify this even further by saying 'Concert G', reminding them that 'your' key is the actual key. In this situation the soprano sax player might say 'it's in my A, that's concert G'.

This may sound complicated! But, in any particular group, you'll only have to establish that you're an 'in C' instrument once. And I strongly suspect the question is about the actual key of the song, not the mechanics of your instrument. Is this a real situation or a hypothetical one?

If the question is about whether you have a transposing instrument (or not) your answer would be 'I'm in C' or 'I'm concert pitch'. Your soprano sax playing friend would answer 'B♭'. This might occur if someone's handing out sheet music and needs to know whether to give you a transposed copy.

If the question is about what key you prefer to play a song in (or about what key you're already playing it in!) you'd answer 'G', 'D', 'A minor' or whatever it was. You might clarify this even further by saying 'Concert G', reminding them that 'your' key is the actual key. In this situation the soprano sax player might say 'it's in my A, that's concert G'.

And, particularly perhaps as it's an ethnic instrument, the question might really be 'what keys are you comfortable playing in?' If remote keys are an issue, say so.

Is this a real situation or a hypothetical one?

If the question is about whether you have a transposing instrument (or not) your answer would be 'I'm in C' or 'I'm concert pitch'. Your soprano sax playing friend would answer 'B♭'. This might occur if someone's handing out sheet music and needs to know whether to give you a transposed copy.

If the question is about what key you prefer to play a song in (or about what key you're already playing it in!) you'd answer 'G', 'D', 'A minor' or whatever it was. You might clarify this even further by saying 'Concert G', reminding them that 'your' key is the actual key. In this situation the soprano sax player might say 'it's in my A, that's concert G'.

This may sound complicated! But, in any particular group, you'll only have to establish that you're an 'in C' instrument once.

If the question is about whether you have a transposing instrument (or not) your answer would be 'I'm in C' or 'I'm concert pitch'. Your soprano sax playing friend would answer 'B♭'. This might occur if someone's handing out sheet music and needs to know whether to give you a transposed copy.

If the question is about what key you prefer to play a song in (or about what key you're already playing it in!) you'd answer 'G', 'D', 'A minor' or whatever it was. You might clarify this even further by saying 'Concert G', reminding them that 'your' key is the actual key. In this situation the soprano sax player might say 'it's in my A, that's concert G'.

This may sound complicated! But, in any particular group, you'll only have to establish that you're an 'in C' instrument once. And I strongly suspect the question is about the actual key of the song, not the mechanics of your instrument. Is this a real situation or a hypothetical one?