The correct option is B CH2=CH−S−CH2−CH=CH2
Carbon containing double bond is called vinylic carbon and carbon adjacent to double bonded carbon is called allylic carbon.
In (a) thio group attached only in allylic position
In (b) thio is attached to both vinylic and allylic carbon.
In (c) thio is attached only to vinylic carbon.
Therefore,
CH2=CH−S−CH2−CH=CH2 (Allyl vinyl thioether)