The formula of Chile salt petre is

The appropriate solution is  option B i.e. NaNO₃ is the chemical formula of Chile Salt petre.

Option A i.e. KNO₃ has the  common name-Bengal salt petre or nitre while option C is sodium nitrite and option D i.e. AgNO₃ has the  common name-Lunar caustic.

Hence, the correct formula  is option B.