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(C) The reaction starts with salicylaldehyde ($2$-hydroxybenzaldehyde). In the presence of a base $(HO^\ominus)$,the phenolic $-OH$ group is deprotona

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(C) The reaction starts with salicylaldehyde ($2$-hydroxybenzaldehyde). In the presence of a base $(HO^\ominus)$,the phenolic $-OH$ group is deprotonated to form a phenoxide ion $(X)$.Next,the phenoxide ion acts as a nucleophile and attacks the vinyl phosphonium salt $(CH_2=CH-PPh_3Br^\ominus)$ via a Michael addition,forming an intermediate where the phenoxide oxygen is bonded to the terminal carbon of the vinyl group.This intermediate then undergoes an intramolecular Wittig reaction. The ylide carbon (which is now attached to the phenoxide oxygen) attacks the carbonyl carbon of the aldehyde group.This leads to the formation of a cyclic oxaphosphetane intermediate,which subsequently collapses to eliminate triphenylphosphine oxide $(O=PPh_3)$ and forms the cyclic product,$2H$-chromene.

Product $(Y)$ of the above reaction is

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