Watch the 1 5 1 Structures online guitar lesson by Brad Carlton from Guitar Lab: Open String Universe
This chord is A root 5th root structure rooted off the 4th fret of the 5th string. The 2nd string open is a B which is the b7 in the key of C#. The 1st string open is an E which is the b3 in the key of C#. The total formula of this voicing is 1 b3 5 b7 therefore it is a C#m7. The formula 1 b3 5 b7 is a subset of the following scales: dorian, phrygian, aeolian. The included chart shows this chord/scale relationship for the m7 harmony.