The correct answer is B. mexicana.
To find this answer, we must recognize the subject of the sentence, which is Susana or ella (meaning she). This subject is singular (because she is only one person) and feminine (because she is a girl). Therefore, we choose the adjective that is singular (meaning it has no "s") and feminine (in Spanish, adjectives in their feminine form typically end in "a" while masculine adjectives end in "o"). Thus, your answer is mexicana, or option choice B.
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