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 3rd fret of the 5th string. The 2nd string open is a B which is the 7 in the key of C. The 1st string open is an E which is the 3 in the key of C. The total formula of this voicing is 1 3 5 7 therefore it is a CMaj7. The formula 1 3 5 7 is a subset of the following scales: ionian, lydian. The included chart shows this chord/scale relationship for the Maj7 harmony.