is most likely in the Key of D Major
Notes in D Major:
A, B, C#/Db, D, E, F#/Gb, G
Chords in D Major:
D, Em, F#m, G, A, Bm, C#dim
This key can also be known as B minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.