The molecular formula of potassium nitrate is:

The correct option is B

${\mathrm{KNO}}_3$

Explanation for the correct option:

Option B: ${\mathrm{KNO}}_3$

1. Potassium nitrate is ionic salt.
2. Potassium nitrate contains potassium, ${\mathrm{K}}^{+}$ ions and nitrate ${{\mathrm{NO}}_3}^{-}$ ions.
3. Potassium nitrate is formed by reaction of one ${\mathrm{K}}^{+}$ ion with one ${{\mathrm{NO}}_3}^{-}$ ion.
4. Chemical equation - ${\mathrm{K}}^{+}+{{\mathrm{NO}}_3}^{-}\to {\mathrm{KNO}}_3$

Hence Potassium nitrate has the molecular formula ${\mathrm{KNO}}_3$ i.e option b is correct.