Bed:
The lower surface of a brick laid flat is known as the bed. The bed is the horizontal surface of the brick that rests on the mortar when it is placed in a wall. It is the part of the brick that is in contact with the mortar and provides stability and support to the structure.

Brick Laying:
When bricks are laid flat, they are placed horizontally with their long sides parallel to the ground. The bed of the brick is then in contact with the mortar, while the upper surface of the brick is exposed. This method of brick laying is commonly used in wall construction.

Arris:
The term arris refers to the sharp edge or corner of a brick. It is the meeting point of two adjacent faces of the brick. The arris can be either a vertical arris or a horizontal arris, depending on the orientation of the brick. In the case of a brick laid flat, the arris would be the vertical edge of the brick.

Header and Stretcher:
The terms header and stretcher are used to describe the position of a brick in a wall. A header is a brick that is laid with its short end facing outwards, perpendicular to the wall. It is usually used at the ends of walls or at openings such as doors and windows. A stretcher, on the other hand, is a brick that is laid with its long side facing outwards, parallel to the wall. It is the most common type of brick used in wall construction.

Correct Answer:
In the given options, the lower surface of a brick laid flat is known as the bed. The bed is the horizontal surface of the brick that rests on the mortar when it is placed in a wall. It is the part of the brick that is in contact with the mortar and provides stability and support to the structure. Therefore, option 'B' is the correct answer.