What caused North Korea to begin to push back the UN troops in the war at the Yalu River? A. China entered the war.
B. Soviet Union entered the war.
C. MacArthur was relieved of his duties.
D. Kim Il-Sung was removed from power.
The answer is A. China entered the war

The Korean War (South Korean: Korean: 6.25 전쟁, 한국전쟁; Hanja: 韓國戰爭; RR: Yugio Jeonjaeng, Hanguk Jeonjaeng; North Korean: Korean: 조국해방전쟁; Hanja: 祖國解放戰爭; MR: Choguk haebang chŏnjaeng, "Fatherland Liberation War"; 25 June 1950 – 27 July 1953)[44][45][c] was a war between North Korea (with the support of China and the Soviet Union) and South Korea (with the support of the United Nations, principally from the United States). The war began on 25 June 1950 when North Korea invaded South Korea following clashes along the border and insurrections in the south.[47][48][49] The war ended unofficially on 27 July 1953 in an armistice.

Korean War

In South Korea: (6·25 전쟁, 한국 전쟁)

In North Korea: (조국해방전쟁)Part of the Cold War and the Korean conflict

Clockwise from top:

A column of the U.S. 1st Marine Division's infantry and armor moves through Chinese lines during their breakout from the Chosin Reservoir UN landing at Incheon harbor, starting point of the Battle of Incheon Korean refugees in front of a U.S. M46 Patton tank U.S. Marines, led by First Lieutenant Baldomero Lopez, landing at Incheon F-86 Sabre fighter aircraft

Date25 June 1950 – 27 July 1953[note 1]

(3 years, 1 month and 2 days)Location

Korean Peninsula, Yellow Sea, Sea of Japan, Korea Strait, China–North Korea border

Result

Military stalemate

North Korean invasion of South Korea repelledUS-led United Nations invasion of North Korea repelledChinese and North Korean invasion of South Korea repelledKorean Armistice Agreement signed in 1953Korean conflict ongoingTerritorial

changes

Korean Demilitarized Zone established

North Korea gains the city of Kaesong, but loses a net total of 3,900 km2 (1,500 sq mi), including the city of Sokcho, to South Korea.[10]

https://en.m.wikipedia.org/wiki/Korean_War

A

Explanation:

describe a little more.