The correct answer is: A depression on a face of brick.
A frog is a depression on the bed face of a brick that helps to distribute the weight of the brick above and to improve the bond between the brick and the mortar. Frogs can be either rectangular or triangular in shape, and they are typically located in the center of the bed face of the brick.
The other options are incorrect. An apparatus to lift a stone is called a crane. A vertical joint in a brickwork is called a perpend. Soaking a brick in water is called wetting.