The quote below comes from the tonkin gulf resolution:

whereas the united states is assisting the peoples of southeast asia to protest their freedom and has no territorial, military or political ambitions in that area, but desires only that these people should be left in peace to work out their destinies in their own congress approves and supports the determination of the president, as commander in chief, to take all necessary measures to repel any armed attack against the forces of the united states and to prevent further aggression.

source:

what led congress to pass this resolution?

evidence of chinese support for north vietnam
a successful viet cong offensive against saigon
reports of north vietnamese attacks on u.s. ships
the election of a communist government in south vietnam

Correct

reports of North Vietnamese attacks on U.S. ships

Further details:

The Gulf of Tonkin Resolution was a measure that allowed the US President to make military actions, like increase troops, without formal declaration of war.  It led to huge escalation of US involvement in the Vietnam War.

The Gulf of Tonkin Resolution, passed by Congress in August, 1964, after alleged attacks on two US naval ships in the Gulf of Tonkin.  The key wording in the resolution said:

Resolved by the Senate and House of Representatives of the United States of America in Congress assembled, that the Congress approves and supports the determination of the President, as Commander in Chief, to take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.

That resolution served as a blank check for President Johnson to send troops to whatever extent he deemed necessary in pursuance of the war.  Between 1964 and the end of Johnson's presidency in 1969, US troop levels in Vietnam increased from around 20,000 to over 500,000.

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Explanation:

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