Answer:
D) either 436 Hz or 444 Hz
Explanation:
frequency of the tuning fork, Fâ = Â 440 Hz
frequency of the piano, Fâ = ?
Beat frequency, F = 4 Hz
Beat frequency is given as the difference between the frequency of the two instruments  and it is given by;
F = Fâ - Fâ           or     F =  Fâ - Fâ
Fâ = F + Fâ          or     F - Fâ = - Fâ
Fâ = 4 Hz + 440 Hz   or    4 - 440 = - Fâ
Fâ = 444 Hz         or    - 436 = - Fâ
Fâ = 444 Hz        or     Fâ = 436 Hz
Therefore, the frequency of the piano is 444 Hz or 436 Hz