While at "el correo" (the post office), the action you are most likely to do is:
C. echar una carta
This corresponds to mailing a letter, which is a primary function of post offices. The other options pertain to different locations or activities. For example, "sacar un libro" (checking out a book) is related to a library, "cobrar un cheque" (cashing a check) is related to a bank, and "llenar el tanque" (filling the tank) is associated with a gas station.