Oxygen is far more electronegative than carbon and so has a strong tendency to pull electrons in a carbon-oxygen bond towards itself. One of the two pairs of electrons that make up a carbon-oxygen double bond is even more easily pulled towards the oxygen. That makes the carbon-oxygen double bond very highly polar.

So, In CH3CHO +I group is present which has a tendency to withdraw electrons, so that the negative charge on carbonyl oxygen becomes increases and positive charge on carbonyl carbon decreases and reaction becomes more difficult and makes it less reactive towards nucleophillic addition reaction while in HCHO,+I group is not there to withdraw electrons. so it will react with HCN very easily.

b)Chloroacetic acid is strong acid then acetic acid because in chloro acetic acid, chlorine atom attached to acetic acid due to its high electronegativity weakens the O-H bond and Also stabilizes the anion making H+ to go in easy maner but in case of acetic acid it is hydrogen hence it is not possible to donate H+.

PKa = - log10 Ka

PKa value depends upon the strength of acid. If acid is strong than PKa value would be less and vice versa.So PKa-acetic acid is 4.23 and chloroacetic acid is 2.3.

c)Alpha hydrogeninaldehydesis more acidicbecause of 2 reasons: 1-Strong electron withdrawing effect of carbonylgroup,which makes release ofalpha hydrogeneasier. Inaldehydes other than an alkyl group there is a hydrogenatom bonded tocarbonyl carbon.