Final answer:
The primary policy proposal of William Jennings Bryan and the Populists in the 1890s was to expand the money supply by including silver in addition to gold. This policy, known as the free silver movement, was designed to create inflation to aid struggling farmers and workers. Despite their efforts, the Populists' influence declined after Bryan's defeat in the 1896 election.
Step-by-step explanation:
In the 1890s, William Jennings Bryan and the Populists' primary policy proposal was expanding the money supply by including silver as money. This stance was also known as the advocacy for the free silver movement or the silver standard. The Populists, along with Bryan, believed that the coinage of silver, in addition to gold, would lead to inflation which would in turn raise prices for farm goods and help farmers as well as the working class to pay off their debts.
The Populist Party supported Bryan in the 1896 presidential election partly due to his stance on this issue, which resonated with their broader goal of financial reform aimed at aiding struggling farmers and workers. Despite their efforts, Bryan lost to William McKinley, and the nation remained on the gold standard, signaling the decline of the Populist Party as a national force.