What is the major difference between one-party and two-party systems?

That there is only one party... A one party system is where there is one group of political views, this is how it was in nazi germany and in the soviet union when it was a communist nation. A two party system is when there are, like the name implies, two parties. The unites states is a two party system, republicans and democrats. There are also smaller parties such as the green party and libertarian party but it is mainly just the two. 

I’m pretty sure that the answer is b.