Why was mao zedong's communist china ready to use military tactics against the troops of us general douglas macarthur in korea?

There were several reasons which made Mao Zedong to use military tactics against the troops of US general Douglas MacArthur in Korea. The US attack on communist North Korea and heading of US forces towards Yalu river which borders China and North Korea worried China about  protecting themselves from what they called "armed aggression of Chinese territory". The Mao's ideology of Capitalist-communist rivalry also made him to go against the US forces as he thought US wants to end Communism in North Korea and later in China. Mao also knew that if US began to intervene in domestic affairs of China, it will boost morale of anti-revolutionary people in China. With these reasons China became offensive and attacked South Korea on 26th November, 1950 and inflicted surprised casualties on US forces.

i would say your answer would be the second option.

hope this !