Since, the molecular formula of D thus, D is HCOOH (formic acid). D is obtained by the acidification of
C, so, C is sodium formate (HCOONa).
Thus, A must be formaldehyde (as it undergoes Cannizzaro reaction with a strong base).
Thus, A = Formaldehyde (HCHO)
B = Methanol
C = Sodium formate (HCOONa) D = Formic acid (HCOOH)