a) alkali , NaOH.
b) when the ph of mouth goes below 5.6 , tooth decay starts . tooth enamel is made of calcium phosphate which does not dissolve in water but gets corroded when the bacteria in mouth produces acids by the degradation of sugars.. it can be prevented by brushing with toothpastes which are generally basic in nature after taking food so that the acids are neutralised.
c)bee sting contains formic acid or methanoic acid whcih causes burning pain . rubbing of baking soda causes neutralisation reaction on the stung area gives relief as the methanoic acid is neutralised by baking soda.