Please help!?!? needs to show work too.

In order to solve problems like this, you need to know that whenever
there are waves involved ...

          (wavelength) x (frequency)  = speed of the wave .

Both of these problems are about sound, so you'll need to know the speed
of sound.  I'm going to use 340 meters per second. You should look it up to
see if that's a reasonable number.

1). (wavelength) x (frequency)  = speed of the wave

The wavelength is given, and I picked a number for the speed.

         (0.667 m) x (frequency) = (340 m/s)

Divide each side by 0.667 m :    Frequency = (340 m/s) / (0.667 m) = 509.7 Hz.

2).  I picked a number for speed, and two frequencies are given.
We have to find the wavelength for each frequency.

20 Hz:
(wavelength) x (frequency)  = speed of the wave

       (wavelength) x (20 Hz) = 340 m/s

Divide each side by  20 Hz:        Wavelength = (340 m/s) / (20 Hz) = 17 m

16,000 Hz:
(wavelength) x (frequency)  = speed of the wave

   (wavelength) x (16,000 Hz) = 340 m/s

Divide each side by 16,000 Hz:  Wavelength = (340 m/s)/(16,000 Hz) = 2.125 cm

I also want to tell you how much I like the wavy appearance of these
questions about waves in the picture you attached !

The answer should be a.