The correct option is: might

• Uncertainty: The statement doesn't provide a definite prediction about rain, only a possibility. "Might" captures this uncertainty better than the other options.
• Recommendation: The sentence advises taking an umbrella, implying not being caught by surprise even if the potential for rain is uncertain. "Might" aligns with this intention.
• Other options:
  • "Must" implies certainty, which might not be accurate if the chance of rain is unknown.
  • "Will" also suggests a strong possibility, potentially stronger than intended in this context.
  • "Can" simply indicates the possibility of raining, without the emphasis on taking precautions.

Therefore, "might" accurately conveys the potential for rain without overstating the certainty, making it the most fitting option to complete the sentence and maintain the intended meaning of the advice.