A tuning fork vibrates with 4 vibrations in 0.8 seconds. Its frequency is

Frequency is the number of waves that travel through a specific place per second. The frequency unit is Hertz (vibrations per second).The time it takes the Wave to complete one full oscillation or cycle is referred to as the time period. The SI unit of time is the second (s).

The following is the relationship between time period and frequency:

Time period $\left(T\right)=1 / f$
Number of vibrations =4
Time for 4 vibrations $=0.8\sec$
So the time for 1 vibration$=0.8/4=0.2\sec$
Time period $\left(T\right)=0.2\sec$
Frequency  $\left(f\right)=1/T=1/0.2=5 \mathrm{Hz}$
Hence, option 1 is correct.