A tuning fork and a piano emit f1 = 523 Hz and f2 = 527 Hz, respectively. What do you hear

1. A sound at 2 Hz disappearing every millisecond.

2. A sound at 1050 Hz disappearing every 2 seconds.

3. A sound at 1050 Hz disappearing every 4 seconds.

4. A sound at 525 Hz disappearing twice per second.

5. A sound at 525 Hz disappearing 4 times per second.

you will hear beats with a frequency of 527-523Hz or 4Hz

this means that the sound will fade with a frequency of 4Hz, or 4 times per second. Ans 5 is correct; the sound will have a frequency of 525Hz and disappear 4 times per sec (we call this a beat)