Answer:
pitch is the quality of a sound governed by the rate of vibrations producing it; the degree of highness or lowness of a tone.
fundamental frequency. For D4 it is at 293 Hz roughly. For both the guitar and the piano, they are exactly the same!
the first couple of harmonics. they are at (293 x 2 =) 586 Hz and (293 x 3 =) 879 Hz. the harmonics are at the same frequencies for both the guitar and the piano show that both instruments are playing the same musical note.
the relative strengths of the first few harmonics. The guitar has a taller/stronger fourth harmonic and the piano has a taller/stronger second harmonic. this shows that although the musical note played in both cases is the same, they are being played by different musical instruments.
hope it helps!