Final answer:
The Chinese civil war was suspended due to the Japanese invasion of China, beginning with the Manchurian Crisis in 1931 and escalating into a full-scale conflict by 1937, which led the Nationalist GMD and CCP to form a United Front against the external threat. The correct answer is option A.
Step-by-step explanation:
The Chinese civil war was suspended because of the external threat posed by Japan. In 1931, following the Manchurian Crisis triggered by the alleged Japanese terrorist attack on the railroad, Japan invaded the region, which escalated into the Second Sino-Japanese War in 1937. The civil war between the Nationalist GMD government and the Chinese Communist Party (CCP) subsided as both factions realized the necessity to form a united front against the common enemy. The civil war was officially paused after the Xian Incident in December 1936, when Chiang Kai-shek was taken prisoner and coerced into halting the conflict to collaborate with the CCP against Japan's aggression.
This suspension allowed the two Chinese factions to create the Second United Front, aimed at combating the Japanese invasion. Nevertheless, the rivalry persisted beneath the surface and the civil war recommenced after the defeat of Japan in World War II.