When transposed up a tone, the passage will be in the key of

The passage is written in treble clef with no sharps or flats in the key signature, which indicates C major. The melody moves stepwise through notes of the C major scale and resolves on C.

Transposing up by one tone (two semitones) shifts every pitch up by a whole step. The tonic moves from C to D, giving the key of D major. D major has two sharps (F# and C#) in its key signature.

To verify: C + 1 tone = D, so C major transposed up one tone = D major.