Option (A) is correct. Public opposition of the Vietnam war had made Johnson unpopular so he decided not to run for re-election in 1968.

Further Explanation:

Vietnam War (1955-75):

Vietnam war was a communal war between South Vietnam and North Vietnam. South Vietnam was supported by soviet union and North Vietnam was assisted by the USA. It was regarded as a proxy war between the USA and soviet union.  

Effect of the Vietnam War:

Following were the adverse effect of the Vietnam War:

• The number of American deaths was around 58,220.

• The report of the Department of Defense stated that the USA spent around $168 billion in the Vietnam War.

• American society was divided into various groups of activist supporting the war. It created social tension in American society.

Johnson’s government (1963-69):

L.B Johnson was appointed as president after the death of President Kennedy. Johnson immediately contributed to the Vietnam war. The Vietnam War had resulted in the loss of thousands of American soldiers and the heavy cost of war. American society started opposing the decisions of the president and started criticizing the government’s actions.

Therefore, Johnson became very unpopular among the USA citizens and decided not to run for the re-election in 1968.

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Answer details:

Grade: Senior School

Subject: History

Chapter: American Wars: Vietnam War

Keywords: Why did President Johnson decide not to run for reelection in 1968, Public opposition, Vietnam War, made, unpopular, public, support, Vietnam War, had made him unpopular, already, served, two, terms, US president, speech, Democratic National Convention, prompted, violence.