i thought it was d7 but the fifth note(A) is missing
It alludes to being D dominant 7. The full chord would be D F♯ A and C(♮).
Often in a chord of three or more notes, some are omitted, for various reasons. Here, the A - 5th of the chord - is omitted, as actually it is sounding as a harmonic of the root - D. It's the 2nd harmonic, and therefore is present anyway - want it or not. That apart, the root's needed to establish the main name, the 3rd as major/minor, and the 7th (here ♭7) as opposed to any other 7th note, so the 5 (A) becomes somewhat redundant.
Oftimes, in a jazz situation, with root note provided by bass, there's only the need to play 3 and 7, which is what frequently happens.
Sometimes we have to guess what the functional harmony is from incomplete information.
This is an easy one. The three active ingredients of a D7 chord are present, the root, major 3rd and minor 7th (forming a tritone with the 3rd). The 5th in triad-based chords is generally just 'filler', it adds to the richness of the texture but is functionally dispensable.
Just F♯ and C would still have been a pretty strong clue (though not absolute proof) that the harmony is D7. D and F♯ would imply D, but not D7.
When you play a D on any acoustic instrument, the characteristic sound of that instrument comes from the relative loudness of the various overtones/harmonics.
Regardless of instrument, any single D note will have the 5th as its second overtone (the first is an ocatave). This means that whether you want it or not, the fifth is there if the fundamental is there.
So, if you or someone else plays D, the A is there automatically. Notice that this effect applies to other harmonics so when notes are played simultaneously as a chord, the sound is much more complex than you may think. For example if you play all the harmonics of a note (perhaps electronically) without the fundamental note itself, they will add together to make the fundamental sound! This has even been exploited with old-fashioned church organs to produce lower notes than would seem possible just from the existing pipes!