What note is played on the D (4th) string at the 5th fret?

The note played on the D string at the 5th fret is G. When you press down on the 5th fret of the D string, you are effectively raising the pitch of the open string (which is D) by a perfect fourth interval to reach G.

In the context of the musical alphabet, moving up from D, the notes are D (open string), D# (1st fret), E (2nd fret), F (3rd fret), and F# (4th fret), culminating in G at the 5th fret. This systematic movement along the fretboard corresponds to each fret representing one half step. Therefore, reaching the 5th fret on the D string leads you to the note G, making this the correct answer.