What did the populists suggest as a solution to the "currency issue" ?

The bitter controversy surrounding the issues of "free silver" and "sound money," so central to the 1896 campaign, has proved difficult for historians to explain. Partisans on both sides made exaggerated claims of the impact monetary policy could have on the nation's economic health. They implied that coinage of silver (on Bryan's side) or adherence to the gold standard (on the Republican side) was the single key to prosperity--and sometimes to the nation's honor. 

The answer is d. spain.