This is a complex type of exercise, involving not only English, but an understanding of the US government as well.
Here are my answers:
I don't particularly like 2, but it's the only logical place the phrase fits.
A: The Congress writes laws. It is its essential function. That would be the first thing a person would say about it. It is elected by the people voting in their states (for senator) or their districts (for house member). They are not elected by "the full party membership." Everyone can vote in a general election. You do not have to be a member of any political party to vote.
B: There are no mentions of parties or other "organizations" within Congress in the Constitution. Only that there will be a Speaker of the House. I wouldn't say that this goes against the "spirit of the Constitution," but this is the best answer here.
C: The British Parliament is much more party-oriented, apparently.
D: The contrast between party and other organization above makes this answer sensible. Party influence wanes and other types of organization grow.
E: It is the leaders of each party in each chamber of the Congress that are elected by the members of that chamber in that party.
F: The House has 435 members.
Student or Learner