The frog of a brick is the indentation on the underside of the brick. It is used to improve the bond between the brick and the mortar. The frog is generally kept on the bottom face of the brick in a brick masonry. This is because the bottom face of the brick is in direct contact with the mortar, and the frog helps to create a stronger bond between the two.
The other options are incorrect because they do not provide a good surface for the mortar to bond to. The top face of the brick is smooth and does not have any indentations, so it would not create a strong bond with the mortar. The shorter side of the brick is also smooth and does not have any indentations, so it would not create a strong bond with the mortar. The longer side of the brick has some indentations, but they are not as deep as the frog, so they would not create as strong of a bond with the mortar.