Final answer:
The word that could best replace 'vigilant' without changing the overall meaning of the passage is 'mindful'.
Step-by-step explanation:
The word that could best replace 'vigilant' without changing the overall meaning of the passage is mindful. 'Mindful' has a similar connotation of being attentive or aware, which aligns with the idea of being vigilant in this context. The other options - careless, thoughtless, and hostile - all have negative connotations that do not fit the meaning conveyed in the passage.
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