D7 chord inversions
D7/F#, D7/A and D7/C are the first, second and third inversions of the D7. This means that the bass tone is shifting from D to F sharp, A or C.
Theory and information
All these chords consist of the notes D, F#, A, C. The difference is that the order of notes shifts. F# is the bass note in the first inversion, A is the bass note in the second inversion and C is the bass note in the third inversion.
For D7/F#, also possible is 2X0212. D7/C can also be played as X30232, but the fingering in the diagram above without doubling of the D note on the 2nd string is generally more natural when D7 is the underlying context. Also notice that D7/C# is playable but will sound dissonant because of the clash between the C and C# notes.
Alternative chord names
D7/F# is identical with D7/Gb.
D7/C is identical with D/C.