The leader of a faction within the Guomindang kidnapped Jiang and held him until he agreed to ally with the Communists against the Japanese. Why would this leader take such a drastic step?

the correct option is: a liberal.

the kind of person that is most likely to hold a left-wing political ideology is liberal because they have in common that they support human individual rights and liberties. also, they both have politically and economically centrist.

moreover, a liberal believes that everyone should have equal rights as well as opportunities to succeed in life, and the freedom to make their own decisions as long as they don't attack the rights of other people. all people deserve the same opportunities and basic needs.

i believe it would influence their own way of understanding things. it shall be a foundation where they can draw or start an argument.   historians are also  scientists that  have their tools of analysis and expertise to unlock a variety of difficulties such as relationships of man, origin, politics, economy, and culture. based on facts and understanding on these areas, they may shade light to extract ideas and develop options to validate a hypothesis or debunk it.