As Matt L. said, this is pretty clearly E minor (natural minor, i.e. [Aeolian mode](https://en.wikipedia.org/wiki/Aeolian_mode)). The A chord can be understood as a [borrowed chord](https://en.wikipedia.org/wiki/Borrowed_chord) from the nearby [Dorian mode](https://en.wikipedia.org/wiki/Borrowed_chord):

    X:1
    L:1/4
    M:
    K:C
    V:2 clef=treble name="Aeolian"
    e, ^f, g, a, b, c d e "C"[e, g, c]
    V:1 clef=treble name="Dorian"
    e, ^f, g, a, b, ^c d e "A"[a, ^c e]
    %

This major IV degree in a minor mode is an extremely common borrowed chord. A great example is Stairway To Heaven, where it is used both to convey a sort of folky mood (Dorian and Mixolydian scales tend to sound a bit celtic on guitar IMO), and to weave in chromatically descending lines.