Which of the following resonating structures of 1-methoxy-1, 3-butadiene is least stable?

⇒ Structures (a) and (b) are quite stable because every atom has complete octet⇒Structures (c) and (d) are less stable than (a) and (b), because in these every atom does not have complete octet.⇒Structure (d) is stabilized by resonance, which is not possible in (c).      Therefore (c) is least stable.