you would use a comma after.
example:
dear Mrs.Smith,
I was pleased to meet you.
sincerly, NK
Answer:
Sorry but I'm pretty sure the answer is A. Colon :)
I has a similar question:
It is important to use which of these kinds of punctuation after the greeting?
the comma
neither of these
either of these
the colon
I got 100 percent! So it might be different for you but for me it was colon!