Why did Southern states fear Missouri's entry into the Union as a free state? They wanted to maintain their majority in the House. They opposed equal representation in the Senate. They were on the verge of gaining a majority in the House. They did not want free states to have a majority in the Senate.

I believe it's because they did not want free states to have a majority in the Senate. (Your fourth answer). This is due to the fact that if they were outnumbered, they wouldn't have the power to keep their interests in trade. They were perfectly fine with having equal representation, they just didn't want free states to have a majority.