The C natural above the A implies an Am chord (A C E), even though the E, in this case, is missing. This is consistent with the key of G major, because the sequence of chords built on the G major scale is G major, A minor, B minor, C major, D major, E minor, F# diminished, and back to G major again.
(The F# diminished triad is actually contained within a D7th chord, so you don't really have to worry about it.)
In this case, the C natural is harmonically correct. But as one of my music theory professors said, "First, you learn the strict rules of harmony. Then, whenever you break those rules, be sure you know why you are breaking them." I would not use a C# in that position unless I had a particular reason for doing so.