Final answer:
The Populist Party declined due to a shift from an agricultural to an urban population, driven by technological advances and a significant loss in the 1896 election.
Step-by-step explanation:
The Death of the Populist Party occurred because of a change in population from a mostly agricultural, or d) Agricultural population to a mostly urban population. During the late 1800s, the United States saw a rapid population growth, with a significant shift from rural areas to urban centers. This urbanization was driven by various factors including the Industrial Revolution, which introduced improved farm equipment and technology that decreased the need for agricultural labor. Furthermore, the Populist Party's decline was hastened by political events, such as the loss of William Jennings Bryan, who held ideals similar to those of the Populists, in the 1896 election to William McKinley.