Chlorine exists in two isotopic forms, Cl-37 and Cl-35 but its atomic mass is 35.5. This indicates the ratio of Cl-37 and Cl-35 is approximately
Chlorine exists in two isotopic forms, Cl-37 and Cl-35 (isotopes are atoms of same elements with a different mass number, i.e. only no. Of neutrons are different.)
Now to calculate the average atomic masse we have to multiply atomic masses with their respective relative abundance percentage (presence) in nature.
The relative abundance of Cl37 is ~24%.
The relative abundance of Cl35 is ~76%.
Thus average atomic mass= 
= (37 × 0.24)+(35 × 0.76)
=8.88 + 26.6
= ~ 35.5
Hence, I the ratio of Cl-37 and Cl-35 is approximately 8.88: 26.6, which approximately comes down to the ratio 1 : 3
It can also be proved by calculating Average atomic mass
As 
= 
=35.5, which is as given.
Hence, (c) 1 : 3 is the correct answer.
Other 3 answers (i) 1:2
(ii) 1:1 and (iv) 3;1 are all incorrect options.