Answer:
Hola amigo!
Step-by-step explanation:
Translated to "Hello, Friend!" Though both could work, this option distinguishes that you're writing a letter to a certain recipient (your friend) and thus is the BEST option.
C. ¡Buen día!
Since it is a matter of responding to a "formal" email, idioms or barbarities should not be used. Option B could be, but I would use it more on cards.
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