The best comparison from the options you've provided would be B) they are mostly democracies like the United States.
Here's a brief explanation:
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A) they have presidential systems unlike the United States: This is incorrect as many countries in Spanish South America do have presidential systems similar to that of the United States.
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B) they are mostly democracies like the United States: This is accurate as most countries in Spanish-speaking South America are democracies, although the quality and stability of these democracies can vary widely.
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C) they have parliamentary systems unlike the United States: This is incorrect because many Spanish-speaking countries in South America have presidential systems rather than parliamentary systems.
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D) they do not have national legislatures like the US Congress: This is incorrect as most countries in Spanish South America do have national legislatures, often with a bicameral system, similar to the U.S. Congress.
So, option B is the most accurate statement about the comparison of government systems.