How many number of neutrons are there in ${}_{17}^{37}Cl$?

1. 20

The correct option is A 20
Chlorine has two isotopes: ${}_{17}^{35}Cl$ and ${}_{17}^{37}Cl$.

The atomic number of chlorine is 17 and mass number is 37 The number of neutrons is equal to the difference between the mass number of the atom (M) and the atomic number (Z).

Therefore, the number of neutrons present in tritium $=37-17=20$.