(a) Salicylic acid gives effervescence with NaHCO3, which is a characteristic feature of carboxylic acid. Thus, it ensures the presence of a carboxylic group in it.
(b) It doesn't give silver mirror with Tollen's reagent because it doesn't have aldehydic group.
(c) It gives red colour with neutral FeCl3 as it contains enol group i.e., group
i.e., =∣C−OH group