In which major key would you find one flat?

F major is the key that features one flat in its key signature. In the context of Western music theory, the key signatures represent the collection of pitches that form a scale.

The presence of one flat in F major indicates that the note B is lowered by a half step (B♭). This adjustment helps to create the distinct character and sound of the F major scale, which comprises the notes F, G, A, B♭, C, D, and E. Understanding key signatures is essential for musicians as it influences both the harmonic structure of compositions and the requirements for sight-reading music.

In contrast, C major does not contain any flats or sharps, G major has one sharp, and A major has three sharps. Therefore, F major stands out as the only key option here with one flat.