(a) (i) Strength of acid depends on the ease of release of H+ions. Benzoic acid contains benzene ring which is electron withdrawing where as acetic acid contains methyl group which is electron releasing. The benzoate ion resulted from dissociation of benzoic acid stabilized by resonance whereas the acetate ion resulted from dissociation of acetic acid is not stabilized. Therefore, benzonic acid easily releases H+ion than acetic acid.
(ii) In methanal, presence of comparatively bulky group than methanol attached to carbonyl group hinders the attack of nucleophile. Also CH3 group present in ethanol decreases the positive charge on carbonyl carbon by +I effect which is not possiblein methanal. As Nu−attack is favourable with more positive charge and less hindrance at carbonyl carbon, therefore methanal is more reactive than ethanol.
(b) Propanal being an aldehyde when heated with Tollens' reagent to give silver mirror but propanone being a ketone does not.
CH3CH2‌CHO+2[Ag(NH3)2]++3OH−→ CH3COO−+

2‌Ag↓

silver‌mirror

+4NH3+2H2O