What note can be played on the 1st fret of the D (4th) string?

The note that can be played on the 1st fret of the D (4th) string is F#. This string, when tuned to standard tuning, is the D string, and pressing down on the 1st fret raises the pitch of the open string by a half step. In the context of musical notes, this means the open D note becomes D# with one fret pressed, and then another half step up leads to E, and subsequently F and F#.

Understanding the sequence of notes on the fretboard is essential. The notes on the 4th string are D (open), D# (1st fret), E (2nd fret), and F (3rd fret). Hence, reaching the 1st fret indeed gives you the note F#.