Explanation:

Here Thread and String are Synonyms of each other. Then the answer will be the synonym of Cask. 
Cask is nothing but a large barrel-like container used for storing liquids, typically alcoholic drinks. 
Ex : Drum, Vessel, Tank, Barrel, Hogshead, Firkin,...
Here from the options Barrel is the only synonym of Cask.
Thus,  Thread : String :: Cask : Barrel