Answer:
a. Pero
Step-by-step explanation:
Kevin es un hombre grande, pero su hermano es un hombre pequeño.
Pero, meaning "but" in English, is the only Spanish word that would make sense in this particular context.
English translation:
"Kevin is a big man, but his brother is a small man."
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The other answer choices "todos" and "todas" translating to "all/everybody" in English, and "chico" translating to "boy" in English, would not make sense when placed in the sentence.