The most common and appropriate reply to "JazakAllah" is "Wa Iyyak" (وَإِيَّاكَ).
Here's a more detailed explanation:
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"JazakAllah" meaning: "May Allah reward you [with] goodness."
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"Wa Iyyak" (وَإِيَّاكَ) meaning: "And to you as well." This is the simplest and most widely accepted response. It essentially returns the well-wishing.
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"Wa Iyyakum" (وَإِيَّاكُمْ) meaning: "And to you all as well." Use this when addressing a group of people.
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A more formal reply: "Wa Antum Fa Jazakumullahu Khairan" (وَأَنْتُمْ فَجَزَاكُمُ ٱللَّٰهُ خَيْرًا) meaning "And you too, may God reward you with goodness." This is a more complete and formal way to respond.
In summary, while "Wa Iyyak" is the most common and readily understood response, "Wa Antum Fa Jazakumullahu Khairan" offers a more elaborate and respectful reply. Choose the response that best suits the context and your relationship with the person who said "JazakAllah."