Answer:
At the Post Office (el correo) you can buy a stamp (named in spanish as sello, sello postal or estampilla).
A sello is generally used as a proof of prior payment for shipments made by mail. This is in the form of a sticker and is usually adhesive, or directly printed.
Therefore the right answer is B:
El Sello
The other options: El jabón (the soap), la pasta dental (the toothpaste) and la pelota (the ball), are incorrect because these items are not sold at the Post Office
