B. The Long March became a symbol of Communist heroism and persuaded many Chinese to join Mao.
The Communist army suffered hundreds of thousands of losses during the Long March, so answer D is far from correct.
There was no battle at the end of the Long March -- so A is incorrect as well. The Long March (which actually was a series of marches) actually served as a path of retreat for the Communists, to avoid being overrun by the Nationalists. They would survive, regroup, and come back to fight later.
And so answer C is incorrect also, because Mao's stature as a leader increased following the Long March, and history would turn in the direction of the Communists in the time that lay ahead.