Test analysis: "A single spark can start a prairie fire" refers to the revolutionary development trend under the theory of armed separatism of workers and peasants, "Stop the civil war and unite with the outside world" is the slogan during the anti-Japanese war, "All reactionaries are paper tigers" and "Carry the revolution to the end" is the slogan of overthrowing the rule of the National Government in the three-year liberation war. In a word, these slogans all reflect the anti-imperialist and anti-feudal themes and belong to the category of democratic revolution. The answer is D.

Comments: This question is more difficult. The key to solving the problem is to understand all the slogans mentioned in the problem as a whole and avoid looking at the problem one-sidedly. This question mainly examines students' understanding ability and requires students to master relevant historical facts skillfully.