The correct choice is
D. 22 Hz and 42 Hz.
In fact, the beat frequency is given by the difference between the frequencies of the two waves:

In this problem, the beat frequency is
, therefore the only pair of frequencies that gives a difference equal to 20 Hz is
D. 22 Hz and 42 Hz.
As it is given that Bulk modulus and density related to velocity of sound

by rearranging the equation we can say

now we need to find the SI unit of Bulk modulus here
we can find it by plug in the units of density and speed here

so SI unit will be

SO above is the SI unit of bulk Modulus