Consider your own personality, interest and strength, such as having a deep interest in mathematical theory research. Those who have a certain foundation at the undergraduate level can choose basic mathematics, and those who are interested in computer programming and mathematical application can choose computational mathematics or operational research. Of course, graduate students mainly write and publish papers. If they are interested in education, they can choose normal mathematics.
There is also a major in statistics. If you are interested in finance major, but want to take a postgraduate exam in mathematics, you can choose statistics major, which has a greater relationship with finance major than other majors.