Chlorine has two naturally occurring isotopes, ${}^{35}Cl$ and ${}^{37}Cl$. If the atomic mass of Cl is $35.45$, the ratio of natural abundance of ${}^{35}Cl$ and ${}^{37}Cl$ is closest to:

The correct option is B $3:1$
The percent abundance of $Cl-35$ and $Cl-37$ is equal to 100.

Let abundance of ${}^{35}Cl=x$ and abundance of ${}^{37}Cl=100-x$.

As we know,

Atomic weight of ${}^{35}Cl=$ 34.9689

Atomic weight of ${}^{37}Cl=$ 36.9695

Atomic mass of $Cl=$ 35.45

Average atomic mass = $\frac{34.9689\left(x\right)+36.9695(100-x)}{100}=35.45$

$x=76\%$

Abundance of $Cl-35$ = 76%

Abundance of $Cl-37$ = 24%

Hence, the ratio of their natural abundance is approximately equal to $3:1$.