Escribo because you are talking about yourself
I would say A because if she is suprised that he only says dear than it would more than have to be A
It means "when is the birthday of your best friend?" amigo is for a guy friend and amiga is for a gal friend.
the way u respond is "<span>El cumpleaños de mi mejor amigo (a) es <span>...."
and fill in the date.
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