Hey There!
let see the definition of law of multiple proportions when two eliments combine to form more than one compound the mass of one element which combine with mass of other element is will always be a fixed ratio ,accordingly answer is first option.
Answer A : N2O , NO , NO2
Temporary hardness can be dealt with using boiling water(Calcium hydrogen carbonate) but Permanent won't so you need to use washing soda instead(sodium carbonate) or Ion exchange resin
You should study geography or sociology but sociology is more about people