Polk started out by trying to buy the land. He sent an American diplomat, John Slidell, to Mexico ...

from PBS:

Polk started out by trying to buy the land. He sent an American diplomat, John Slidell, to Mexico City to offer $30 million for it. But the Mexican government refused to even meet with Slidell. Polk grew frustrated. Determined to acquire the land, he sent American troops to Texas in January of 1846 to provoke the Mexicans into war.

When the Mexicans fired on American troops in April 25, 1846, Polk…declared, "[Mexico] has invaded our territory and shed American blood upon American soil," and sent the order for war to Congress on May 11.

The act was a questionable one. Many Northerners believed that Polk, a Southerner, was trying to gain land for the slaveholding South. Other Americans simply thought it was wrong to use war to take land from Mexico.

…Ulysses S. Grant…would later call the war "one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory."

Cite this source: (PBS.org)

Which quote from the text BEST helps explain a reason why the United States was justified in fighting the Mexican-American War?

Determined to acquire the land, [Polk] sent American troops to Texas in January of 1846 to provoke the Mexicans into war.
Polk…declared, "[Mexico] has invaded our territory and shed American blood upon American soil," and sent the order for war to Congress on May 11.
Other Americans simply thought it was wrong to use war to take land from Mexico.