The Populist Party was a third party created due to the discontent of farmers against the major political parties (Republican and Democratic parties) because of the Gold Standard issue, and because they felt ignored. This Party is often regarded as a failure because it focused on one issue (to increase the money supply) and tried to gain the support of farmers and workers, but the workers' wants and needs were different than those of the farmers, so the Populist Party didn't had all the support it needed to succeed and ended up being absorbed by the Progressive Party. Although the Populist Party didn't succeed, it helped creating awareness of the farmers conditions, so that the Republican and Democratic parties could also focus on that side of the population.
The correct answer here is the option D.
Tet offensive was a major offensive undertook by the Viet Cong along with the North Vietnamese People's Army of Vietnam and it showed the shocked South Vietnam and its American allies that the war was actually far from over. This also led to the loss of the support for the war in the States.